IIRC, the trader behind the starting point in the hideout has some, the trader in the safe zone on the swamps, and finally, the trader in the city on the catwalk (outskirts). Might be more, I wasn't bothered to check, by the time you make half a circle around the maps you usually already have modern/composite parts for the gun = you reached the endgame. What I did check, however, is that each individual trader seem to stop selling those after you buy a few (cumulative runs), or I just got a really bad RNG.