http://www.sdp-si.com/eStore/CoverPg/Gears.htm
This is where I order all my gears from. Large selection of miter gears or what ever you want. Not everything is in stock all the time and the prices are decent but very easy website to use and order from. Just make a shopping cart and order. No minimum of an order as long as you pay by credit card.
Every other good gear supplier I have found is a quote/contact to order. That's just too time consuming to me.
There is also
http://www.mcmaster.com/#standard-gears/=jhalkw but this is a limited selection.
I've bought miter gears pretty cheap once before from ebay. Hard to find what you want though.
-Jeff