Also find a lot of 5.56mm in destroyed MG turrets - 20ish in each if you're lucky. And later raiders, Gunners and sometimes super mutants start carrying assault rifles a lot. And found a fair bit of 5mm as I've levelled up (once ran into three super mutants who each had a minigun lol). Cricket who sometimes shows up at Bunker Hill usually has a bit, as does Kleo in Goodneighbour - but might be by level?
Would love to find more .308 so I could upgrade my combat rifle to the .308 chambering.