Common Gear set. Lego Technic compatible.

40 Tooth Gear.

24 Tooth Gear.

16 Tooth Gear.

8 Tooth Gear.

If you don't have a 3D printer but you're interested in getting a set we can print you one at http://www.badbrick.com "Like US" https://www.facebook.com/badbrick

In MakerWare, I modify the "Medium" settings:
I use the Medium Profile with minor adjustments.
Quality: Layer Height = 0.12
Infill: 20%
Shells: 2

All other defaults are the same.

The finer you print them the better they will be.
Triangle file is your friend.