The is a downloadable kit that you cut out and make. Once you've payed online, you print out the pages of this model onto thin card stock paper, follow the illustrated instructions and make your a working model.
You can get this model individually or as part of their 'Gears Collection' package deal.
For those in the U.S.A., don't worry about the prices being in Great British Pounds, PayPal will convert the Pounds into Dollars for you.
Here is a link to a page with the Rack-and-Pinion paper model.