Your Example is a single stage reduction, with 21V that will be really fast. We use a single stage with speed 900s, but on 12V and even then if we dont put our armour on it pulls a wheelie every time.

http://www.computerclub.nl/uploads/msrconcept.jpghttp://www.computerclub.nl/uploads/msrconcept.jpg

You can find gears in several locations, one of them is RS Components.

http://uk.rs-online.com/web/http://uk.rs-online.com/web/