I have an old Mantua 4-4-0 that was missing the shaft and was pleasantly surprised that even after a couple acquisitions, I was still able to get one from their eventual successor: Model Power. Perhaps you can holler up Hornby (the new owners of Rivarossi, IIRC) and see if they'll be able to get you one...
And never underestimate the power of Walthers to come up with oddball stuff -- you just have to call them, not depend on the catalog...