I've been ordering my bulk ammo from Aim Surplus for quite a while and have always been happy with their service. They run out of stock from time to time due to the high demand but they currently have 5.56 M193 brass case non-corrosive ammo in stock again. I just made an order to replenish my supply. It's the same brand I'm already using (Prvi Partizan) and I've been very happy with it. Right now you can order 1000 rounds for just under $360 plus shipping.
Note that if your AR15 or other rifle is chambered for 5.56 ammo you can shoot this M193 5.56 ammo in addition to .223 ammo, but not the other way around. If your rifle is chambered specifically for .223 you should only shoot .223. Your chamber type will probably be stamped somewhere on the receiver or barrel depending on what type of rifle you have.