Well said JoshYup, eBay is the best place. A simple search for "1/64 Kyosho" will turn up hundreds of results.
What's special about Kyosho's rubber tires are that they come in different width and tread patterns depending on the car/wheel type and the cars' respective era. For example, race cars will have slick tires, modern sport cars will have complex tread patterns, and older cars has narrow tires and simple tread patterns. The same goes with the wheels too as they offer distinctive wheels original to the cars, whereas many other manufacturers will re-use the same wheel design. It's little details like these that makes Kyosho so special.