I think pewter is very UNsuitable for a wheel. It is very soft, will not wear well, and may even distort just under pressure of handling. Think of its ''normal' uses as a decorative casting material for jewelery. The stuff used for harder items like beer mugs or plates I suspect is a much harder alloy.
Either machine your wheels from a solid like brass or steel, or look for commercial wheels.
If you really want to cast them look for a jewelery casting business. I use one in Rhode Island, but he will no longer take new customers. I make a master pattern he makes a rubber mold for usually $25-$40, then cast parts in bronze are $2 -$10 based on weight. A model train wheel would likely be in the $2-$3 range.