I've ordered parts through Car Quest, with good luck, for my '48 4400. Also, I've gone to our local heavy truck and drive train shop and matched parts up with newer stuff. If all you need is rebiud parts, check with any parts house that's been around for a while. A few catalogs I have, "Brothers Trucks" and "the Truck Shop", list the 1 ton parts you're looking for.
Good luck,
Scott

Scott Ward

2 1948 1.5-ton Loadmasters
The red one and The snow pusher
1 1950 3100
1 1955.1 Chevy 6500
1 1954 Chevy 6500 2-Ton
1 1955 1st Series COE 5700
1 1963 K20 (454)
1 1964 C10 (350)
1 1951 1.5-ton Dump Truck
1953 and a 1956 Ford F800

Raising a teenager is like trying to nail Jello to a tree!