Hi Brian, In addition to the excellent advice above... I've always found my local NAPA to be an excellent source of parts for the bigger older trucks. If they don't have the parts, they should be able to tell you more about the parts or steer you to someone in your area that does. Hopefully, you live in a more rural area with an older NAPA that deals with farm trucks and big trucks. If you do live in such an area, there should be a truck shop locally (Again, the guys at NAPA probably do business with one or more of these shops and can tell you where they are). These places are also a wealth of information, if not also of parts, etc. If it is an older, farm-oriented shop, you never know what they might have stashed in the attic ...
Places like these should also be able to steer you to shops that reline clutches and brake shoes.
If you live in a more urban/suburban residential area (with lots of mini malls, Starbucks, etc) ... all bets are off. Still try NAPA but if their clientele consists mainly of white collared millennials looking for oil filters for their Teslas ...
But if the parking area is full of beat up vans or pickups with ladder racks, fuel tanks and other tools hanging off them ... you are at the right place!