Don't know where to get them locally but DSG Arms has a great price on them: Cavalry Arms Mark II Lower Receiver Black
If I wasn't broke, I would buy a couple more while they can still be had.
I have handled them and they are put together very well and their specs are top notch.
Here is some good discussion on them: Accurate Tool Firearms - M4Carbine.net Forums
I built this super-lightweight 16" carbine using a stripped Carbon-15 Upper and a Cav Arms Lower. The result is 5 lb. 1 oz. (without magazine). It handles and shoots great.
The C-15 Upper assembled exactly like an aluminum one (except there is no forward assist or ejection port cover).