From experience on a variety of 50-80 year old vehicles, there is no single source and I look EVERYWHERE. The internet has been a game changer, but I tend to find parts on all of the above - I like Chevs of the 40s; The Old Car Center in Southern BC (being up on the wrong side of the 49th, it's still shipping to me); and eBay can be useful for obsolete parts. In Canada KMS Tools even has some stuff for Chev trucks. I have even found parts on Amazon (usually by searching a part number). Don't forget places like Summit Racing, Speedway and other standards as well because sometimes they actually have direct replacements. Yes, I have also found some parts at NAPA.
Some direct sale manufacturing shops will have parts as well.
Sometimes you have to keep checking. Recently I was having a heckuva time finding a remote Bendix vacuum brake servo for our first fire truck (its a F### but the part is actually a GM part used on GM trucks) and all of sudden, a new one popped up on eBay. It is installed and working. Similarly, running boards for my 3/4 Ton 3600 (longer than the 1/2 tons, and very hard to find - 1/2 ton running boards are typically mislabeled on vendor sites as fitting the longbox when they do not).
The thrill of the chase is part of the fun.
If you're in a better location than I am, you may have wreckers and swap meets. I have none of those!
