A good place to start is
www.questformadness.com (The RC Part)
When you read the section on wheel mods, and attempt your first one, you will see it's really quite easy.
As for where to get the wheels themselves, I recomend checking out the nearest toy box to see what junky cars (or trucks) that are close to the same size can do without their wheels!
Aside from that, hit the clearance toy isle in your local X-Mart with your new "Modding-Enhanced" vision and see what might render some good wheels cheap.
Also do a search in these forums right here with the word "wheels" or "wheelset" in it.
I know I personally have posted about half a dozen wheelset mods here with pics and details, if not more.
Good luck! Enjoy!