Controlling Speed and Powering 12V DC Motor with Arduino and Wall Supply
So I'll throw a some quick info at you,


That motor looks to be quite big, it does not give a current spec but I would expect it to draw upwards to 10A when under load. That means any motor control shield or board you buy needs to be able to handle at least the maximum current (Which i am guessing is around 10 Amps).
For a motor that large, it may be difficult to find a cheep power supply for it. If money is not a problem then I'm sure you can find a 12V supply on eBay that will work.
To control the speed of a motor, you need what is called an H bridge or half an H bridge if you only want to go one direction. Ebay again will have a ton of H bridge circuits that you should be able to attach to a PWM (analog out) port on the Arduino.


I would recommend you get some current measurements on the motor you want to use before trying to build a driver system for it...