I use NAPA mostly, we have a commercial acct. there and internet PRO-Link access so we can look up our own parts, prices and if it's sitting on the shelf waiting for me. We get a break on price so it's reasonable for us. Keep in mind the only thing NAPA makes is boxes with their name on them. It is ALL stuff they buy from name brand manufacturers by the trainload and stuff in their boxes. So your actually getting pretty much the best name brand part at the best price for it's quality. (I'm not a NAPA salesmen, sorry for the sales pitch)
I also use Advance Auto, O'Riellys. I use Autozone when I do not have a choice.