Where is the 5.56 ammo?

[Ham Sup Lou]

There's a ton of 5mm, 10mm, .38, and even .308

but for some reason, I can only find the 5.56 from wrecked turrets and through a few vendors.

[ghpstage]

A lot of things in this game don't really start appearing until you reach a certain level, weapons and ammo especially.
Eleanor near Fairline Hill Estates stocks a lot of ballistic ammo regardless of level however, so check her.

[Futa Fanatic]

your lucky i still havnt found any 5.56 ammo -.- got my first 2 guns last week and im above L80 :/

[Chaosium]

High level Gunners use a lot of assault rifles, hence your best chance to find 5,56 is to kill them. Raid the plaza and you should have a stock.

Else just pick Scrounger lvl 3 and enjoy yourself everywhere.