One way to get Prisoners is to go to Online Multiplayer and trade with the people you meet on there. Some are new to the game and will want all the weapons, so you may offer Yarn in exchange for their Prisoners. Some people may also take Gems for the same amount they paid for the Prisoner. Depending on how many times you have used the Prisoner Concession Device, you may pay more than the person you are trading with, so you may get a discount!

Winning Online Arena matches will get you 4 gems (5 if you get MVP). There is also chances a Golden Hat (+10 gems) will appear. This is one way of farming gems needed to unlock prisoners through the Prisoner Concession Device.

You may do Solo or Co-op playthroughs in Normal Mode or Insane Mode to get gems. Collect all the gems you can and Grab the Yarn if possible. If you do this quick enough, you will receive an A++ on that level giving you the most gems you can get from that level. Note that whatever gems you collected so far on a certain will not be given to you again if you do the same level twice.

For certain people this is your best bet. For others it is a last resort. If you have funds in your Steam Wallet that you you would be willing to spend, you could use the Community Market to buy whatever Prisoners you do not have yet. They will be added to your Steam Inventory and will become usable in game.

Here is a picture showing all Prisoners! (Excluding Star Prisoners)

Got any additional tips? Put them in the comments and they may be put on this Guide!

Tip from xXShmegXx: It is a good Idea to get all of one kind of prisoner than getting a bunch of different ones. For Example, if you got a Circle Prisoner, Keep getting Circle Prisoners until you have them all, then move on to another kind.

Tip from Phobia:You can get the same rewards for Offline Arena Matches as well, you would just be facing an AI rather than a person under the same conditions. This is a good idea if you are having problems finding or playing in Online Arena Matches.