I just bought a set of four Kenda Karrier 205/75R15 tires, mounted on new white painted steel wheels, from these guys:
https://www.johnsonssurplus.com/ Shipped they are $90 each, but if you are close enough to drive to White Pigeon, MI, it's $68 out the door. They also have all sorts of RV salvage--I've never seen so many doors, furnaces, AC units, and water heaters--as well as new, hard to find items like exterior corner moldings, hitch replacement parts, brake drums and backing plate assemblies, etc. A lot of this stuff isn't on the website, you have to call them.