Palmetto state armory (ordered 5 psa lowers) and Carricoz (ordered 2 mega lowers) have both been great to work with. For other AR parts Brightflashlights is excellent (best YHM prices ive seen - selling new at used prices and amazingly fast shipping) as well as RRarms and JSE surplus. The other big names are good too (midway, ctd, brownelles, midsouth, natchezss, etc.) and i have heard good things about Buds, Grabagun, and Aimsurplus. Sorry to ramble.
That said, i have had the best luck with individual people (mostly on ar15 and pafoa).