This is a model of the tire that is needed for the wheels of a Tomy OmniBot 2000, OmniBot, Hearoid, and the Robie Sr from Radio Shack.
This was created for the refurbish of an OmniBot. A tire was printed out, molded with silicone, and cast in a urethane rubber. You can see the refurbishment here...
There's also printable rubber filament that may work too.
If you have problems printing it out, you can cut it in half, print out the halves and glue it together. That ended up working best for me.
If this model helps you out, please offer a tip.