I want to repair the Remnants power armor and helmet and I dont have (yet) the jury rigger perk
I know there are merchants that repair you armor but I don't know where they are.
Can anyone please point me in the right direction.

Also, I really want to own a Ballistic fist but I can't buy it nowhere :'(
Tried gun runners, BoS bunker (after the related quests) but nothing.

Who uses this weapon so I can go and "take" it from him/her

Please help me out.

For the the Ballistic fist you can either check back with BoS merchant after 3 days or reload the game after checking her inventory. One of the traders at the 188 can repair, raul, calamity(jacobstown) , ralph(or is it mick), one of the ncr guys at the mojave outpost and old lady gibson, i'm sure theres plently more.

If you're high level sometimes they have ballistic fists at crimson caravan.

Some of the Legion guys have Ballistic Fists, but I'm not sure what level you have to be to start having them show up.