Some pretty good deals here on stripped lower receivers:
http://www.aimsurplus.com/acatalog/Receivers.html
(for those of you who don't know, but are curious, the stripped lower is the only part of the AR-15 that if purchased over the internet, needs to go through a licensed gun dealer--in essence, the stripped lower *is* the firearm. The rest of the firearm can be purchased in just the same way as a a book from amazon).