Rainier Arms is a good bet, followed by DSG Arms. Also, don't forget the PX right here on the Hide which has served up quite a few .308 AR lowers and parts like BCGs of various makes/models in the last month, as have the EE sections on BARFCOM. Short of those outlets, GunBroker, Armslist and other sites have plenty to offer...if your wallet is fat enough to stomach some of the prices which are just now beginning to come back down from orbit.
What, specifically, are you looking for in terms of make/model of lower, bcg, etc.? That piece of info alone may go a long ways in helping you to track something down.