A couple of places that may have what you need.

Harbor Freight link: http://www.harborfreight.com/material-handling/solid-flat-free-tires/6-semi-solid-wheel.html

Amazon had several brands on this page: http://www.amazon.com/s/ref=nb_sb_noss_2?url=search-alias%3Dlawngarden&field-keywords=6+inch+wheel&rh=n%3A2972638011%2Ck%3A6+inch+wheel

If you want to shop at a retail store, you can try Tractor Supply, Atwoods, Harbor Freight, and Northern Tools. They all carry those types of tire/wheels.