This is what I found when randomly exploring commonwealth (I didn't even do anything to the main story aka go to Diamond City). I dicited to make this guide a day after I found it so thats why it's coloured.

You will find Full set of any thing in there depends on what level you are! You need to be atleast level 28 and up to find any X-01 armor in the world.

You need to go to this location on the map (shown by my character arrow).

It's West, South-West of Costom House Tower.
If you travel on top of Costom House Tower, it's trough this alley way.
It's in side of or on top of this building is the armour, 35 Court, there will be two 10mm turrets protecting the entrance.
In side the building there will be two turrets and protecttron gaurdian, you will have to go to the elevator on the left.
Though the the elevator ride to the left and throuhg the hallway.

Here you will fight two tought robots Sentry Bot and Assultron, they will come out the right and the left rooms, kill them and push the the buttons that are in the rooms from witch the robots have come out of.

And there you go, you have a brand new X-01 armour and free a Fusion Core, witch every nameless NPC will comment on.