Small Parts has them, I don't know how big you can get them. Call them tommorow, and ask them for the sprockets that go with the hub attatchment for the skyway wheels, tell them you're with robotics they will know what you are talking about. You can have them shipped 2-day.
hub Part number- FIR-DC01
they talk about the hub in this post:
http://www.chiefdelphi.com/forums/sh... way+sprocket
There is a picture of it that Matt Reiland posted.
We just ordered some of the same sprockets
They aren't normally in the small parts catalog, they are in Appendix D