I second the Surplus Ammo and Arms lower, made by Aero Precision. Aero makes lowers for other big name players too, just with different roll marks applied (obviously).
My lower that is purchased from them is terrific, makes a tight fit to the Aero upper and I was able to finish the complete lower for under $250 with a Daniel Defense premium lower parts kit and an ACE Skeleton stock. The best deal going, IMHO