Where to get replacement wheels?

Honestly, the easiest way is to just buy mainlines and swap wheels.
Sometimes you can go on the Hot Wheels Customs groups, where the builders make a lot of custom stuff. They occasionally dump lots of plastic wheels in those groups. Never hurts to ask.

Yep, wheel swapping. It's hard to find plastic wheels online

I was gonna say just go to your local retail store and buy a bunch of mainlines and open them up to get the wheels. This is what we refer to as "wheel harvesting"

I usually buy my wheels as rubber.

Good old wheel farming - read about it

Overall, the forum collection on wheels & axles has a long list of articles about getting wheels, modding wheels, and everything in between.