Where do I find replacement wheels?

peg warmers usually, it's cheaper to buy mainline cars on the pegs. 

Some of mine were mystery cars, Body was Atroucios but wheels and axle are sweet.

As these guys have said, it's easier to find peg warmers and fantasy cars that have decent wheels than it is to find a specific vendor for "speed" wheels. I look for the cars I like first, then look to see if there is a peg warmer with the same size wheels. (For instance, the wheels on the Speed Driver wrench car are a direct swap for the Lethal Diesel. So are the Retro Active wheels.). True, you spend two bucks to build a $1 car, but then you're also doing all the other work like adding weights, etc., so by the time you're finished, you have maybe a $3-$5 car, just all custom.

I have swapped wheels with "peg warmers" on a couple of my builds. I just thought that I was missing out on some top secret racing wheels that the seasoned builders were using. LOL! Thanks for the advice!

agree about buying two or 3 cars to build one

I have bought boxes of yard sale cars over the years

sometime ya get lucky with those, too