Go to Pyro, they are everywhere. You will need to first take everything off the NPC, and then use the new loot screen to get the undersuit. This is the easy part, because just having the undersuit will not let you equip clothes and armor. You will need to put the clothes you want to wear in an SCU box, then drop the undersuit on the ground close enough you can interact with both the suit and box, but far enough that it will take a few milliseconds to move from one to the other. Now, for the hard part, of which you will also need a low pop server so your inventory is responsive ebough to open the box while your character moves to the undersuit. Google how to do this in a video.