Several sources in the back of Shotgun news sell ready-prepped once-fired USGI 5.56 brass. Tumbled , deprimed and decrimped. Some of the big reloading houses (Midway , Wideners , Natchez , etc) have it from time to time too. Do a Google.
Do you want processed brass rtl or just once fired? Natchez has what seems to me, a pretty good price on some new Lake City 5.56. 119.00/1000. I bought some and it seems to be pretty good and consistent. However, I took plenty of time to prep it(perhaps too much, but that's just me).
Just wondering. BTW, it's LC 11 so pretty new stuff.
Or you could buy some loaded ammo, go have fun shooting it and then keep the brass to reload. I even tell my non-reloading buddies to save me their brass if they shoot it. So far I've gotten 500 pieces and don't even have my M4 built yet.