One you get into the BoS ship, go to the merchant. You have to get through the Master locked door to access the inside.

There is a guass canon, x-01 Helmet and a bunch of other goodies.

Of course, it's stealing .... So pop a stealth boy and have fun!!

Helpful bump

you can also find a full set of x-01 power armor underneath good neighbor during the dig quest in a side room you have to use the robot to blow into. its guarded by a mirelurk king though.

Mine wasn't x1 I guess it scales with level what level were you when you got it?

i think i was like level 53 when i did that quest.