Where can I find the right arm for the X-01 armor? (Fallout 4)

Random. Depends on the level.

Each power armor (or rather segments on the frame) spawns randomly, depending on the level. Yesterday I found an X-01 set without legs at a sentry post. (was level 31). At 40m I found a complete set on Bonnie's quest (or whatever her ... in short, a ghoul from good neighborliness). Thus, you simply cannot find a "hand". Walk around the places where Power Armor spawns and collect the set "in parts". Or swing to a higher level and find a complete set right away. You can also put a mod for crafting power armor. Crafting X-01 parts is very expensive and skill-demanding, so it's not particularly cheating. (I can't give a link, I've seen it somewhere. Look on the site)

Ctpannik
Thank you!

I got stupidly full set!

I just got a complete set of X-01, type IV in the building "35 court". At level 42, there was somewhere

as an option...

at the South Boston checkpoint take the full set

I found a full set of armor at the National Guard Training Grounds and there was also a set without legs in the armory.