Here are some gears for the downloading.
http://www.cbliss.com/inventor/Parts/PowerTransmission/
Martin Spur Gears 169 sizes
14.5 and 20 degree Pressure angle
http://www.cbliss.com/inventor/Parts...Spur Gears.zip
I have downloaded the gears from the link above and they seemed to work great for me. All the gears in our transmission are from Martin. But I found this link after I had already created gears. Phil Dolan from Autodesk told me about the link, I also noticed Dez mentioned it.
The best thing you can do is find a spec sheet on the gears you need to create!