Try ebay. I dunno cheap gears aren't always good. You could always goto www.4wheelparts.com for a cheap ring and pinion they sell the genuine brand those are as cheap as they come.
They ae cheap and don't usually last a long time.
They sell a set of gears with master installs for $429.

Check Randy's ring and pinion they always have thier Yukon brand gears on sale they are usually Motive brand gears and are a decent set of gears. That's what I am using.
http://randysringandpinion.com/default.asp?pid=2&pageid=154
They have gears on special for $486 with master installs at randys as well. I would pay the extra $50 for a decent gearset than going with the elcheapo genuine gears.