I don't have one source for USGI mags, since I just shop
around, and when I find a decent price I buy some. As
for capacity, for plinking, I like the 30 rounders, and
they look so "Cool". For hunting, I prefer the 20 round
mags. I think my case spare mag pouches are loaded with
2 20 round mage, loaded with 55 gr. Ballistic tip hunting
ammo, and 3 30 round mags loaded with FMJ ammo, for plinking.
For whatever that is worth.
Squeeze