Call and find out your wheel size, diameter, offset/backset, and lug pattern. Once you have those dimensions you can easily look around ebay, craigslist, and various other places. I'd suggest staying with
Alcoa Wheel Products Commercial Vehicles Heavy Duty Truck Trailer Bus Motorcoach R/V brand wheels, but there are several others of decent quality. Check out
CONTINENTAL WHEEL COMPANY COMPTON, CA (310) 603-1014 too.. they're made right here in the Good ole USA. I'm not sure how abundant used wheels in your size are going to be however.. you may end up having to buy them new.