Hoplite, Inc. are selling current Colt lowers, they also sells uppers and complete new LE models, see their ads in the Shotgun News.
Hoplite doesn't have a website, you have to call them to find out what are available.
PO BOX 6599
Shepherdsville, KY 40165
Phone: (8 to 5 pm EST)
You're correct, Colt lowers and parts are expensive. I built last year an all Colt M16A2 complete upper half, M16 BCG/ CH, stripped Match Target HBAR lower, LPK with large FCG pins and M16A2 buttstock kit and my total cost was almost the same as buying a complete Colt LE 6721 A3 Tactical Carbine at that time.
Current lowers are flat black color, but if you could get lucky and find a pre-ban with Colt grey color your upper would match better.
Also check your local dealers, I found my stripped lower from my dealer.Photo link of my Colt A2 build