I go to http://www.palmettostatearmory.com
They get it out quick, and at a decent price. It maybe reloaded, but I'm not 100%. It is deffinately dependable ammo to run in most AR's with a 1:7 twist. Not that you can't run it through a 1:9 twist barrel, but it was designed for the 1:7 twist barrel.
For instance, a 300 round bulk pack of M855 is about $110.