A few years ago, I went through that same drill with my '65 GMC 1.5-ton.
The good news is that the one-piece wheels you want *are* out there!
The even better news is that you are going to have a great experience looking at lots of cool old trucks, bread vans, busses in lots of cool, scenic locations (i.e., junkyards, barn yards, truck lots ...) in your search! Along the way, you will probably get to meet many wonderful fellow inhabitants of this beautiful planet we live on (snakes, bees, various rodentia, guard dogs, cattle ...). Cool, huh??
Trucks to look for as likely donors are the '70's vintage F-series (600?) Ford trucks. U-Haul trucks, especially. Similar vintage school busses and bread vans are good, too. Airport shuttles ...
Some junk yards even have piles of wheels you can pick through for extra fun!! (hint -- the ones you need are at the bottom!! Where the snakes and brown recluses live
Mine came from a U-Haul truck and a school bus as I recall. But keep the convo going here. Many of us, in similar searches, often come home with wheels that appeared to be the ones we wanted in the junkyard but weren't. So somebody else's rejects/extras may be what *you* need. For example, I have a complete set of 10-lug 16-inch Budd wheels someone offered me (and I gladly took home) only to discover they were hub piloted (and I need stud piloted ...
IMPORTANT NOTE -- Remember to take your tape measure with you on your search. And have a cheat sheet on you with the measurements you need! Don't try to keep them in your head ...
You do have a search ahead of you! Keep it fun!