Academy Sports carries a house-brand ammo called Monarch. They carry two grades - the bottom end one (blue box) is made by Bernaul and sells for like $8.00/20, and is steel-cased, dirty and inaccurate (at least in my M1A). Might be good for blowing holes in mud or something. The better grade (green box) for like $12.00/20 is actually made by Prvi Partisan, has brass cases and is Boxer primed. It's acceptably accurate, at least as accurate as most milsurp stuff I've tried.
There seems to be plenty of the lower grade, but the higher grade has been out for a while. I guess that not even a shortage of ammo will cause folks to spend money on crappy stuff.