|The following is based on Creation Club content and has not been confirmed by canon sources.|
It can be found on a table next to a power armor helmet inside Galaxy News Radio.
Perk, I assure you the Power Armor the Brotherhood left behind is perfectly clean. I washed the insides myself.
I know you've mentioned multiple times your concern of whether or not they urinate inside the suits. I'm not going to get into the particulars of that with you, but even if they did, it doesn't matter. The Doc will tell you it's completely sterile. You are not going to turn into a molerat if you touch the stuff.