Classic Parts = Best catalogs and website. Very poor technical and customer support. Not all parts work for the year truck listed (listed by their vendors, I've learned,)
LMC = Great technical support and people, Poor website and catalog is ok. Customer support for returns (not had to test yet)
Summit Racing = Great all around, but may have to spend a little more time finding you part in the vast list of options. Many transactions with hot-rod and original parts.
StoveBolt Parts Wanted forum = Awesome all around!!
Ebay = Great for hunting for OEM parts...can take a while, but worth the hunt.