Where are some good traders for this? Going to stop by Diamond City as my next side quest takes me close to there. I'm really struggling to find 5.56mm ammo. Finally found my first assault rifle, but only have about 60 5.56mm rounds. and they are all from destroyed turrets lol

There was a wandering trader outside of Diamond City, plus the traders inside there. Haven't looked form 5.56mm NATO from them yet though.