1) Damage Upgrade mod. Inside the factory, first mission. Lab 2, storage 3.
2) Rate-of-Fire Upgrade mod. Inside the police station armory
3) Reload Speed Upgrade mod. Locker near the police morgue.
4) Ammo Capacity Upgrade mod. Helipad storage at Sarif HQ. Locked behind a level 5 security door, below the helipad.
5) Ammo Capacity Upgrade mod. In the alley north-east of Sarif Headquarters, behind the level 3 electronically locked storage unit, where the player could possibly meet Ezekiel Sanders.
6) Ammo Capacity Upgrade mod. Underneath the floor in a hole in Derelict Row, in the area next to a sewer cover. This can easily be found during Cloak & Daggers sidequest.
7) Reload Speed Upgrade mod. On the 4th level of a fire escape outside Derelict Row, in front of the blue alley door. Requires jump enhancement.

8) Ammo Capacity Upgrade mod. Possible reward for Cloak & Daggers sidequest when crossbow is given up. Can be obtained twice if you let Jenny arrest O'Malley.
9) Laser Targeting System mod. Quest reward for completing the Lesser Evils quest.
10) Armor Piercing System mod. In Jensen's apartment, stash behind the TV.
11) Silencer mod. First visit to Seurat.
12) Burst Round System, Damage Upgrade mods. First visit to Grayson.
13) Target-Leading System mod. Gang Leader near Highland park.
14) Laser Targeting System mod. FEMA camp, ground level, first little trailer outside the main building.
15) Ammo Capacity Upgrade mod. FEMA camp, the warehouse with the elevator to the lower levels, in the small security building with a guard and openable window.
16) Reload Speed Upgrade mod. FEMA camp, Level -4, in the security office near the elevator to Level -5.

1) Reload Speed Upgrade mod. One locked behind a level 2 secured garage left of the Hung Hua Hotel's first floor.
2) Target-Leading System mod. One in a low-level locked garage west of The Hive.
3) Rate-of-Fire Upgrade mod. One inside The Hive's interrogation room (accessible by air vent).
4) Burst Round System mod. At the east of the highest level (3) of Kuaigan District, an upgrade can be found on a balcony (on the west side of the street), just before the stairs going to the lower level (2).
5) Rate-of-Fire Upgrade, Explosive Rounds mods. First visit to Peng Xin Hao in Alice Garden Pods.
6) Rate-of-Fire Upgrade mod. Hidden above Peng Xin Hao inside Alice Garden Pods (reachable by stacking boxes, breaking the weak wall in section 04, or through use of the jump enhancement).
7) Armor Piercing System mod. Located in the Alice Garden Pods under the south side stairs.
8) Ammo Capacity Upgrade mod. Found in the east of Youzhao district, on a rooftop behind two boxes and a grate. A long ladder leads up to the rooftop, nearby is a wall that can be punched out. A gated security door leads to this area which leads to Kuaigan District. This area is also close to the penthouse you're asked to check out in the main quest.

9) Ammo Capacity Upgrade mod. Another one also in Youzhao district west, in a secure storage room in an alley.
10) Damage Upgrade, Laser Targeting System, Target-Seeking System mods. First visit to Lin Fu Ren in Hung Hua Hotel.
11) Ammo Capacity Upgrade mod. Found in Tai Yong Medical Lower City Factory, Maintenance Catwalk Room 2.
12) Reload Speed Upgrade mod. Where you find the eBook Better Than Nature in the Pangu to Tai Yong Medical.

13) Heat Targeting System mod. Tai Yong Medical Penthouse, in a safe behind Zhao's desk (Hacking 5 required, or passcode from pocket secretary in room prior).

1) Ammo Capacity Upgrade mod. In a fenced in area near the helipad on the roof.
2) Rate-of-Fire Upgrade mod. Inside men's restroom on level 6 of Picus TV, immediately before the elevator room.
3) Silencer mod. Found in a drawer behind some boxes in level 3 broadcast room, in front of the turret.

4) Cooling System mod. In the armory of the Picus Sub-Basement
5) Reload Speed Upgrade mod. Inside a locker within a locked office of the Picus Sub-Basement.

1) Ammo Capacity Upgrade mod. Chiron Building helipad, as you step off the helicopter, behind a breakable wall right after returning to Detroit.
2) Silencer mod. Quest reward from Smash the State sidequest during the second visit to Detroit. Obtained by knocking out Jacob White instead of killing him.
3) Ammo Capacity Upgrade mod. Just after confronting Sandoval, there is a breakable wall to your right.
4) Rate-of-Fire Upgrade, Target-Leading System, Target-Seeking System, Heat Targeting System mods. Second visit to Seurat.
5) Explosive Rounds, Cooling System, Armor Piercing System, Target-Leading System, Laser Targeting System, Ammo Capacity Upgrade, Damage Upgrade, Reload Speed Upgrade mods. Second visit to Grayson.

1) Laser Targeting System, Silencer, Target-Leading System, Target-Seeking System, Cooling System, Heat Targeting System mods. Second visit to Lin Fu Ren in Hung Hua Hotel.
2) Armor Piercing System, Target-Seeking System, Burst Round System, Explosive Rounds mods. Second visit to Peng Xin Hao in Youzhao District.
3) Cooling System mod. Harvester Hideout, level -1, in a Level 5 safe in the NW lab.
4) Damage Upgrade mod. Harvester Hideout, bottom level, in one of the cells with a breakable wall.
5) Ammo Capacity Upgrade mod. In the Harvester Hideout, in a Level 5 safe in the living quarters area outside of Tong's office. You can find the code in a pocket secretary on one of the guard's or use an autohack device, if needed.
6) Reload Speed Upgrade, Ammo Capacity Upgrade, Target-Seeking System, Cooling System mods. Visit to Lu Pin Rong underneath Belltower dock.
7) Rate-of-Fire Upgrade mod. Belltower dock, locked room on the left sublevel of the outside area, guarded by a ceiling mounted turret at the far end (relative to the main warehouse), Room A.
8) Damage Upgrade mod. Belltower dock, locked room on the left sublevel of the outside area, guarded by a ceiling mounted turret at the far end (relative to the main warehouse), Room B.
9) Target-Leading System mod. Belltower dock, inside an open shipping container. May need enhanced jump to reach. From the main gate, just in front of the concrete barricades, look left above the Car Pro carports, on top of a stack of shipping containers.

10) Reload Speed Upgrade mod. Belltower dock, level 4 (where sniper is patrolling outside on roof terrace) of warehouse, interior on a shelf. The level below (just over the railing) has an air vent which provides another entry point into the warehouse proper.

Brig
1) Silencer mod. Located in the crate marked "Ammo", near Adam's gear. Will appear only if you chose "Yes" and "Redirect" during opening conversation with Keitner.

Forward Cargo Hold
2) Reload Speed Upgrade mod. Located in the storage room with the elevator, behind the crates.
3) Ammo Capacity Upgrade mod. Open container that is reachable by gantry.

Topside (Ship Deck)
4) Rate-of-Fire Upgrade mod. Inside a locker behind a L2 gate.
Aft Section
5) Damage Upgrade mod. Inside room CMPT-02 B-01, which is locked by a L2 panel.
Loading Bay 2
6) Ammo Capacity Upgrade mod. Located in a maintenance area accessible via ladder and jump augmentation from before the loading bay, or via ventilation shaft from the crane gantry.
7) Reload Speed Upgrade mod. Located in the blue container 3 levels up in the northeast corner of the bay, along with the Launcher Barrel for Garvin Quinn's quest.

Loading Bay 1
8) Laser Targeting System mod. Located inside the suspended blue crate.

Maintenance Level
9) Laser Targeting System mod. Garvin Quinn.
10) Damage Upgrade mod. In a maintenance area accessible through a vent hidden behind a crate at the end of the passageway past Garvin Quinn's shop.

Administration Block
11) Silencer mod. Located in the Communications Security room, inside the area guarded by the lasers. Door code is 1550.
12) Damage Upgrade mod. Located in Commander Netanya Keitner's office, hidden underneath her personal desk. Can hack the door or through the vent, entrance on the west side of L1 Admin offices.
Detention Block
13) Ammo Capacity Upgrade mod. Level 1, along the South West wall is a heavy box covering a vent. Follow to find the upgrade.
14) Target-Leading System mod. Found in the same area as the Ammo Capacity mod on level 1, from there look up you may need the jump enhancement to reach. Follow the new vent and located at the bottom of some stairs.
Maintenance Level
15) Reload Speed Upgrade mod and Target-Seeking System mod. Located behind Garvin Quinn's shop. Can only be accessed once you receive a message from Quinn after finishing Mission 3. Pressing the button hidden behind two wooden boxes will reveal a secret stash.

1) Ammo Capacity Upgrade mod. Immediately after you exit the transport pod from the cutscene, explore the area to find a storage room in the corner with an orange door. May need enhanced jump to get atop the scaffolding to drop down onto roof to enter from air vent, or hack the level 5 door.
2) Reload Speed Upgrade mod. In the dormitory building, past one of the locked bunk rooms (code 1385) on a chair in the back of the room.
3) Damage Upgrade mod. Omega Ranch, in the room where you disable the signal jammer, behind a level 5 security door.
4) Cooling System mod.

1) Burst Round System mod. Panchaea, from the starting area (while facing the entrance to the installation), the upgrade is hidden in one of the large metal containers on the left. It can be identified by one part of the door lying on the ground before it. Jump enhancement or a crate is required to reach it.

2) Ammo Capacity Upgrade mod. Found at Panchaea, in the first building. Go up the stairs heading towards the elevator. From the stairs, facing the elevator, it is on the left about half way down the corridor on top of the very highest scaffolding, on top of a box with a sniper rifle.

3) Cooling System mod. Panchaea, inside one of the rooms along with a heavy rifle. The room has a broken door and large cart in front of it, and can only be accessed via ventshaft.

There are 21 Praxis kits to find in Human Revolution (excluding the Missing Link expansion):
8 in Detroit (4 LIMB)
10 in Hengsha (4 LIMB)
1 in Montreal
2 in Panchaea (2 LIMB)
Also there are 16 Praxis kits to find in Missing Link. Praxis earned in the Missing Link will carry over, but has a special script check to ensure you don't reach Singapore/The Omega Ranch with more than 61 Praxis points in total.

1) Inside a box shortly after exiting an elevator in the Sarif Industries Milwaukee Junction factory. It's fairly hard to miss as it's highly visible and right in the player's path, and the Augmentation tutorial even pops up at that time to discuss Praxis kits. However, if you don't collect it you will miss the chance, as you'll never get to return to the factory later. One may backtrack to this point so long as they have not yet left the factory by means of talking to Faridah Malik.
2-3) LIMB clinic - two can be bought for 5,000 credits each (on his first visit to the LIMB clinic, Adam is given 5,000 credits to spend, effectively making the first Praxis kit free if he chooses to buy one).
4) Detroit Downtown Apartments, third floor of the westernmost building (the one with the security gate), inside an illegal augmentation 'chopshop' with a level 5 security door (code to get in: 2356), then hack the level 2 terminal (8221), then hack the level 1 computer (atcod) for the final code (3663) into the area with the kit).
5) In the sewers beneath Derelict Row. These are accessible from a manhole cover in the bushes along the wall on the right side of the entrance to the first building, or from a manhole in the construction site below the antenna. The kit is in a hidden area accessible by breaking through a wall. Location of wall is shown by Jensen icon in the image. (i.e. the white-bluish arrow at the east end of the map. Enlarge the image for clarity.)
6) Behind a breakable wall in the sewers below the Hung Hua Hotel in Hengsha (from the ladder to the hotel go left and wall is opposite a stack of mattresses).
7) In the bottom of an elevator shaft in Hengsha Court Gardens (access from the southwest roof via a hackable door or breakable wall or through the upper floor vent shaft on the left of the elevator as you exit).

8-9) LIMB clinic - two can be bought for 5,000 credits each.
10) From Bobby Bao - the bartender at The Hive. Either search his corpse, or during his side quest (Bar Tab). Convince Jaya to make the monthly payment, or pay 5,000 credits yourself. Note: after completing the sidequest, you can punch out Bobby Bao to get your 5,000 credits back. Just be ready to run out of there in a hurry...
11) A reward from Malik after doing the side mission (Shanghai Justice) for her.
12) In the Upper City Labs of the Tai Yong Medical building there is a medical room (Laboratory B-2) accessable from the Lee Geng Memorial Labs. This room is located through a door on the top floor of the Lee Geng Labs (left hand side while facing entrance). The Praxis kit is inside a desk drawer in Laboratory B-2.

13) Inside a drawer, in one of the cubicles in the center of the Picus Web workspace, on the same floor as the room 404.

14-15) LIMB clinic - two can be bought for 5,000 credits each.
16) One in Isaias Sandoval's apartment across the street from Adam's place and is only available if Zeke Sanders survived the encounter in Milwaukee Junction. Go to the street alley in front of Adam's place. Climb up the ladder in the alleyway until you reach the building's entrance. Afterwards, walk to your right to find an open gate that was not available before. The Praxis kit is in the main room, next to a dead guard. The kit is connected to two EMP mines (hidden under the dead guard) that should be disconnected before grabbing the kit (if the player does not have EMP resistance). Note that the trap will not be there if Zeke Sanders has been killed prior to this point; as long as he is alive and conscious, regardless of whether he is jailed or set free, the trap and the Praxis kit, will be there. If you kill or render him unconscious (whilst in the apartment and before disturbing the trap), the trap will be automatically disarmed.
17-18) LIMB clinic - two can be bought for 5,000 credits each.
19) On your second visit to Hengsha, a reward from Doctor Wing's side quest (Talion A.D.) in the LIMB clinic.
None.
20-21) There is a LIMB clinic hidden behind a vending machine that can be moved. Two Praxis kits can be purchased there for 5,000 credits each. After completing your goal in the control tower and riding back down the elevator, pass through the door that has newly been opened. In the area labeled "Ring Section" on your map, there's a side path with a vending machine jamming a door. You will likely need the strength augmentation to move the vending machine and get in.

Hei Zhen Zhu
Brig, Floor 4
1-7) Special "Praxis case" near Adam's body armor with 7 Praxis kits inside.

Aft Section, Floor 4
8) Container with a Praxis case inside, it can be seen through the window of the control room with the security camera terminal. This is the second control room past the CIC, not the one with the damaged stasis pod. Can be reached from 1) Floor 4 by breaking the fragile wall of the container (it will kill the soldier standing behind it), 2) Floor -1 by stacking 2 crates or with an upgraded jump.
Rifleman Bank Station, part I
Loading Bay 1
9) In the loading bay's second room, on top of the cargo crane holding the blue container (not the room with the boxguard). Reach it by stacking two crates to reach the ladder in the northwest corner of the very first area after you lower the ship ramp. Climb up and through the duct to come out on the first room's crane. Walk across to the far side and find a duct that brings you out onto the crane of the second room, and the Praxis kit will be leaning against an ammo case.
Admin Sector, Floor -1
10) By default there is a single Praxis kit for sale in Garvin Quinn's shop. Good idea to create a new save game just before talking to him. Activating the Social Enhancer augmentation and using Omega pheromones will drop the price of the standard stock Praxis kit from 5,000 to 2,500 credits, however you will not be able to obtain kit 11 or 12.
11-12) Activating the Social Enhancer augmentation and using Alpha pheromones will convince Quinn to open a completely new 'special' stock with 2 additional Praxis kits, among other things. Special stock replaces the standard, so buy Praxis kit #10 before you convince him. Kits 10, 11 and 12 will be discounted to 3750 credits instead of 5,000 if you have completed the Acceptable Losses sidequest on the Hei Zhen Zhu.
Detention Camp
13) On top of the mainframe room in the center of Detention Block B. It can be reached by jumping from the balcony on Floor 3 near the typhoon turret. The turret can be destroyed with an EMP grenade without causing alarm. To make it to the platform you have to sprint before jumping. It's a bit tricky, but doable after a few tries, even without the jump enhancement augmentation. To get down again just shoot the glass ceiling and fall down to the mainframe room. Alternatively, you can go the opposite route — shoot through the ceiling from within the mainframe room and use the plethora of crates in the area to climb up to the roof.
Restricted Wing
14) Inside the drawer right next to Dr. Tiffany Kavanagh.

Rifleman Bank Station, part II
Interrogation Wing
15) On the topmost platform of the elevator shaft. Can be reached either using the Jump Enhancement leg augmentation or by stacking two heavy crates with Move/Throw Heavy Objects augmentation.
Maintenance Level
16) Near the safe inside Quinn's shop.

http://deusex.wikia.com/wiki/Weapon_modifications_(DXHR)
http://deusex.wikia.com/wiki/The_Missing_Link_walkthrough
http://deusex.wikia.com/wiki/Praxis_kit