DPMS or someon who makes a DPMS compatable. You can use Magpul mags in them and they are sweet and cheap at about 18-19 dollars each. I am in the process of building one on a Tactical machining based 80% lower. They also sell full lowers for 200 and they are very nice.
Lower Receivers & Lower Receiver Parts | Tactical Machining
Lower Receivers & Lower Receiver Parts | Tactical Machining