1954/55 GMC 350 (2-ton) COE: I have usually found wheel bearing, axle bearings, drive-shaft parts, brake parts, and a few other mechanical parts from Rockauto.com and NAPA (especially if I could first find part numbers).
I get part numbers from Master Parts Catalog, oldGMCtrucks/Stovebolt members, Spanky, Stovebolt.com members, and Internet searches.
For other parts:
Jim Carter has usually been my first choice for parts (for over 20 years) - he has been selling parts since the early 70s.
It is unfortunate that his website is quirky - this is the 3rd or 4th generation of his website. The first generation was mirror images of each page in his excellent hard-copy catalog. You had to spend time moving through it, but it was logical and informative.
Myfriendly joke regarding his various generations of "upgraded" on-line catalogs: he hires needy relatives to do web design/programming/testing.
Jim Carter is about as nice a person and as knowledgeable a person/vendor as you will ever meet.
The people who work for him are friendly and helpful.
His parts are almost always "correct", but they have not always been "correct". He almost always makes good on problems/mistakes.
He is an asset to Chevrolet/GMC truck owners/"restorers".
