LOL...what he said!!
Every now and then I see 1/18th scale models on sale for about 10 bucks (roughly the cost of Pegasus or many Aoshima rims), they would be a better source for custom rims than anything else on the market, though if you can find them, Pegasus makes 23" turned aluminum sleeves that make their rims much larger...maybe more like 17's on a 1/20th scale model.