Go to
www.bostongear.com and go to the "Bostpec" tab. They have every gear in their inventory (at least I have always gotten what I have needed) as a solid model in either native form or a generic format (IGES, STEP, PARASOLID, etc.).
I just looked o the site and they do not have inventor parts native, but they do have several generic formats that Inventor should be able to import. Here is a list in order of most robust to least robust:
1. Parasolid
2. STEP
3. ACIS
4. STL
5. IGES
You will get an e-mail with a zipped file containing the solid model.
-Paul
P.S. - I agree with Tristan regarding the gear teeth. However, you should also model a version showing the OUTSIDE diameter of the gear to make sure there are no clearance issues.