You should be able to get a set from wings and wheels. You'll probably have to buy 2.

If you only need one, say to let you make an unassisted aerotow launch from some lonely airport where you have landed out, you can use a temporary wheel. I keep a small wheel and some heavy duty double sided tape in my glider. You can get the wheel and the tape at any hardware store. Be sure to get the heavy duty tape. It's gray, with a red plastic film.

Good luck!