Go to gun shows. Don't buy the first you see, look around for a good deal in there Also, as most AR15's for civilian use are chambered for .223, you don't wanna use 5.56 in it, it'll get rough on the gun Unless it is chambered for 5.56, then have a ball
Wait a few months until the panic buying dies down. Right now any new ammunition you get 'cheap' will probably have something wrong with it. And don't buy ANYBodies reloads at a gun show, unless you don't value your gun or your face.