You're talking complete .308 uppers, which are almost unseen "in the wild" and I have yet to see any of them at a an LGS. You'll find complete .308ARs but the uppers alone are almost impossible to locate, so that leaves you two options - local gun shows, or buy it off the internet. :D
I know the CMMG MK3s have a spotty history but I think they have the majority of the issues fixed over the last year or so. Haven't heard of any problems with the ones that my buddies picked up.
....that said, I'd kill for a complete KAC SR25 with a 20" barrel.... :)) :))