Option 1:
Sabertooth 2 x 25
http://www.dimensionengineering.com/datasheets/SabertoothDIPWizard/start.htm
Option 2:
https://www.pololu.com/product/2393
Option 3:
http://www.robotmarketplace.com/products/roboteq_speed_controllers.html
Other related:
http://www.open-electronics.org/here-comes-the-open-source-snow-plow-robot/
http://www.open-electronics.org/ (cadvision)