You can buy most any and all sizes of scooter and powerchair tires solid and foam filled here.
http://www.electricwheelchairparts.com/foamfilledtires.htm
These are easy to put on, but may be a lil tricky getting the first of the four hub bolts threaded.
Suggestion,
Put a "c" clamp on the side of the rim to compress the hub together to get the first bolt threaded
You can also use a vise (put cardboard in the jaws first so you do not marr the rim.
