I'm surprised they haven't (Jeff or John popped up on this thread yet) I believe it is short for Central Missouri Machine Guns... but maybe I made that up. www.cmmginc.com They make their own as well as resell all kinds of guns/parts for AR's and M16 type weapons. I recently purchased a DPMS upper from them and couldn't be happier with it. One hole 5 shot groups :) But they stock all kinds of goodies. They're in Fayette MO, about 45 minutes North of Columbia I think and a bit west. I actually only have dealt with them at shows, I'll see their shop in April for the 3 gun match there.
You can check out their goods by using the MO for sale thread stickied in the hometown forum for Missouri. They're industry partners on the site, have great service, and seem to be all around good guys. They're probably 3 hours or so from you but if you make a list they can probably have everything you need there when you go and if you time it when there is an event like the MODCC or the 3-gun match you'll have a nice road trip.
If it was me I'd source the parts on the Equipment Exchange... in the AR-15 forum there are lots of threads with parts lists and so forth. Then just go to CMMG for the things you need specifically from them. i.e. if you like one of their uppers and your lower.
Justin