I don't know where you are, but contact tire dealers in your area, especially any who sell tractor tires and they should be able to tell you where to get it done, if they don't do it themselves. I don't know of any (although such may exist) that you can do yourself.
You can also try any shop that deals with forklift tires. Foam filling is very common for industrial use. Know the exact size of your tires as it determines the amount of foam. It's usually better to stop by in person as price can sometimes be negotiated /forums/images/graemlins/smile.gif . It can't be done while you wait as it takes 24hrs+ to cure the urethane, and some shops will only fill on certain days.