Did you try searching in the metric section under gears? There's a drop-down menu and you can browse the "gear blanks" window for the specific blank you need. It also gives you a slection on the center bore.

I would prefer a gear with teeth, just solid so that I can machine it where necessary. sjcrss seems to be making some forward progress on the gear, so we may have more options!

Have you seen these guys
http://www.khkgears.co.jp/khkweb/order/main.do
It looks like they will make custom gears

Looks really handy. I will have to look up the material codes and see if any are useable, such as nylon or delrin.

I am starting my own thread now for pic of what I have come up with so far.

Similar to Cowboy's nitro revo diff idea, but different. Same principle of diff support, just different gear mounting to work with the stock gear.