Suitable motor for robot
You probably want to step up to a larger battery pack than a 9V.  You'll find that the 9V doesn't really supply the current that you need, and will run out very quickly.  You can probably find a small rechargeable pack online or at a local hobby store that will do the trick.

As far as the motor, if the robot is suitably small, I would use a modified hobby servo.  They come in many sizes/speeds/torques, and you will probably be able to find one that meets your specifications.  You can also interface with them using the Servo library on the Arduino.  You can either buy a servo pre-modified, or you can modify it yourself using the instructions found on Acroname, for example. 

For finding these parts and more, I have compiled a list for our undergraduate projects, you can check it out here: Auburn SPaRC Suppliers.  This is a list of common suppliers that we use for many different robotics pieces and parts.