You can't buy them ready made as far as I know and so you need to buy 1/8th donuts and fit them to 1/10th buggy rims after you cut the ribs off.
http://www.rc-direct.co.uk/-c-39_198_205.html
It looks good too because you can make them the same profile as a rubber tyre should you choose to and they work VERY well, better than mini pins on carpet!