I have used quite a few vendors but the bulk of what I bought for my car came from NPD, MU, Autocrafters and WCCC, pretty much in that order in terms of how much stuff. BTW, I do not generally find MU to be expensive as Adam says, in fact they are many times the best price (and I shop everything). You do have to be careful of what you are buying, some places make no bones about selling junk (and not saying so). WCCC will tell you what they are selling you, right upfront, they'll say good if it's good, junk if it's junk (sometimes that's the only game in town), they also tell you they are not the cheapest.
Regards,
Bob