Stripped lower recievers are available from DPMS, Eagle Arms (Armalite), Bushmaster, Olympic, Rock River Arms, and others.
There are several different types of SRs as well: Cast Aluminum, Forged Aluminum, Stainless Steel, Carbon Fiber, and probably more.
To make it easy on you, I would recommend a Forged Annodized Aluminum stripped receiver from DPMS. They are well made and worth the couple of extra bucks.
The cheapest SR I have seen is the Cast Olympic, but I have had troubles with them and would not recommend them.
For more info, see the "Build it Yourself" board.