Re: Tires?
Try Google or some other search engine.
Typically you can purchase trail tires, wheels, and the like at places like Northern Tool (MN), Fleet Farm and similar stores, and many tire retailers will be able to help you get them- they can be ordered if not in stock.
Wouldn't hurt to do a search on web based sellers either. If for some reason you can't locate a retailer in your area, consider ordering off the web.
You can always take tire and wheel assemblies to a service station and pay them a few bucks to mount the tires.