I had similiar problems back and just stayed away from gears. I went with alternatives like belts or something. Best place is to purchase gear boxes that have the configuration you need, but I have a feeling you have other plans for them.
If you don't mind purchasing things via the Internet, try this website, it will send you to some legit site that probably just manufactures gears.www.thomasregister.com
Just get familiar with it and you can find anything. Usually some companies just manufacture one thing and you can get them pretty cheap.
In fact, I was just looking for surface mount LEDs yesterday. I used this website and 'sampled' about 5 parts from 20ish different sites out of 186. So now I have tons of LEDs coming by mail for free, a good way to shop
That's how I get PIC microcontrollers for my projects. Except their new line of them.