There are outfits out there making wheel and tire sets. I bought one set once from MRC. They were very nice and had a comprehensive selection of connectors included so the wheels would fit most any plastic model car. Their only problem for me was the cost. The wheelsets cost as much as a car kit.
In addition there are many aftermarket producers of resin parts out there. Most are based in peoples' garages or basememnts and they come and go frequently. OTOH, Modelhaus supplies many, many replacement parts for old AMT car models. I've been buying from them for 20 years or more and they've saved many a restoration for me.
Of course there are those of us with large spare parts boxes because we've been around awhile. If you know that Car A was built on the same platform as Car B there are usually parts that will be usable in either. Windshields and chassis are some of those easily shared. Older models from AMT days sometimes shared engine mounting sizes and one can switch out the Buick engine from the '40 Ford for the Pontiac engine in the '36.
Replicas and Miniatures of Maryland used to make the best car parts.White resin,almost couldn't tell them from AMT stuff (when amt was a real model company).I havent done cars in a few years so I'm not sure if they are still around.