I've found only 1 mod, and that was for a .357. Where are all the weapon mods?

Finding all mine with the NPC merchants, but they seem quite rare still at level 13.

I really, really need the entire set of mods for the hunting rifle and the silencer for the varmint rifle....and what are the three mods for the service rifle? I have the forged receiver and springs, but have no idea what the third is.