www.ammunitiontogo.com has usually done okay for me on prices. They also go to all the gun shows down here.
I have seen a lot of Federal 223 available for sale at sporting goods stores and gun stores in bulk. I've had good luck with Federal 223 and it was generally competitive. I would avoid steel case ammo unless you just want ammo for blasting. If you shoot it, take care to clean the chamber very well. A gun store in Houston sells 68 Gr Black Hills match ammo that is fairly accurate in my match barreled AR.
I would second another post on Brownells aluminum mags. The few I have work well and seem very well built. I have a number of Magpul P-Mags and have never had any issues with them. They are slick and tough and feed well. If you get a combination of those two, I think you will be well served.
That said, there are lots of USGI mags out there. I am sure others have their favorites. I would avoid any suppliers that are not USGI spec or "cheap" mags. Most of the problems I had with my first AR were due to cheap mags and cheap ammo.