Have you looked into Power Wheels? They are the ones that fit on battery powered toy cars that the kids rise around on.
Here is a quick link to some wheels:
http://www.mypegperegoparts.com/tireandwheels.html
If you yard/garage sale at all look for a used Power Wheel. Mine you it does not have to run, just make sure the wheels are in good condition. I had made my last cart with a set and sold a pair of 10" wheels on these forums for $20. They work great on soft sand and hard pack. You can search my posts under the sale section to see the ones I sold.