You can buy a mounted tire on a rim at most auto supply and trailer supply sources.
My Hunter Compact-II has ST175/80R13's on 4 on 4, 5 spoke, rims that I bought at Arrow Trailer Supply. I have also bought rims w/ tires
for our Scamp
at Pep Boys Auto Supply. There are a number of on-line sources as well such as etrailer.com.