I have bought directly from the following manufacturers...
http://www.sdp-si.com (My favorite)
http://www.bostongear.com
http://www.picdesign.com (Only North American source of 0.7 module gears that mate with the drill motor pinion.)
Here are some other links...
http://www.emerson-ept.com
http://www.martinsprocket.com
http://www.schafergear.com
You can also get gears from Motion Industries (above), McMaster Carr, MSC, Small Parts...but they are being resold from the manufacturers listed above.