You'd be surprised at how many square body parts your local auto parts store has. For example, vent window latch hardware, windshield wiper washer assy, headlight switch, radiators, and more.
And there are still a few of these trucks in junkyards. I found a rust-free inner fender, complete vent window assembly with good rubber, and some trim parts at one near me.
Other vendors I've used:
ClassicParts.com -- decent prices, fair shipping charges, and good tech support
LMC Truck -- OK if you don't need any technical help, but their shipping charges are high. If nothing else, their on-line and printed catalogs with exploded views are really nice.
There's also RockAuto.com. Sometimes they are WAY less expensive than other vendors, and they carry name-brand parts like Moog, AC-Delco, Sealed Power, Raybestos, KYB, etc. Just be careful of the shipping charges, because you could pay separate charges from 2- 3 different warehouses which could nullify your savings.
Speaking of name-brand parts, sometimes Amazon.com has low prices on AC-Delco, KYB etc. along with free shipping. And they are very good about accepting returns.