You can get Motor shields from several suppliers, like this: http://arduino-direct.com/sunshop/index.php?l=product_detail&p=82
If you want to connect to a separate motor driver board and/or other components, you may like a "Sensor Shield" that makes it easy to connect or cable to different components, like this: http://arduino-direct.com/sunshop/index.php?l=product_detail&p=2