Good Thread! I don't normally jump into a discussion first, but am glad to this time
For the nearly 30 years I have been active in this hobby, and the 25 years we have been doing this web site, I have had nothing but outstanding support and service from Jim Carter (Jim Carter's Antique Truck parts). Jim and his staff are active in our hobby as well and do care about us. Jim has also stood by us with support for this web site.
Our biggest supporter was Pete K who owned American Classic. Pete got us through our first big growth spurt by giving us our first dedicated server and then followed up with a lot of support after that. This web site would not exist today if not for Pete. Sadly, Pete passed away a few years back and American Classic was bought up by Classic Parts. Still a good company and worthy of your support.
I've always enjoyed working with Kanter Auto Products, as well. I have not bought anything from them in a few years, but they seemed, like Jim Carter, to be staffed by fellow hobbyists who really knew their stuff. Prices may not have always been the most competitive, but you knew they were high quality, correct and would be what you needed.
There's two (active) for starters.