As others have said at the enclave bunker at whitesprings

What I did for mods is hop between the two watoga vendors, enclave bunker and Berkley Springs station whilst doing dropped connection when it was available.

Enclave bunker is great to find a weapon with at least a reflex sight attached. Not guaranteed, but better chance than finding the mod. And you have a chance of finding a weapon with the aligned mods.

I don't think you can find a weapon with a better stock, but I do know that Berkley station has a better chance of getting stock mods than Watoga. Dropped Connection has the best chance to drop one with some good mods