Well two things.
First, I ordered a new one off of ebay for 25, it was used but in great condintion.
Second, if you havn't already opend it up and looked inside the case, I would do so since it might not be the gear.
After ordering it then trying to fix it I noticed that it wasn't really the gear that was messed up on mine, the problem was that part of the case that held the gear clip into its little hole had broken (I guess when the dude owned it before me)
I ended up using the new gear (since it was newer) and the gear clip off of my old gear since it held tighter and was able to get it to stay put.
When you look inside there you will see the grove hole that the clip is supposed to stay in so the gear will pop out and engage the main gear.
Hopefully yours might be easier then mine, but I hope this helps.