Check on the auction sites. Ton's of interesting things under "scooter wheels", scooter rims, etc.
like these:
http://cgi.ebay.com/Go-ped-Goped-Whe...item3efdf0a924
you can get ideas here to:
https://www.surpluscenter.com/wheels...catname=wheels
Sometimes pulling the wheels off of casters is a good solution.