'51 truck and '63 tractor here - you’d be surprised! For the majority of standardized things (seals, bolts, bulbs, etc) it’s easy enough to find a cross reference table, and just order the part from napa or Grainger.
For more specialized things (like if you break an input shaft), eBay or any number if specialized stores online will have original parts that have just been sitting in a warehouse for the past 70 years. Here’s one of my main sources for the truck: shop.midwestmilitary.com/product-category/m37/
Occasionally I do have to fabricate a replacement.