Micro Servo 9G
The micro servo 9g is a really cheap servo you can obtain from ebay. With some modifications, you can also use it as a really bad geared DC motor. However, this quite fine for many 3d printed devices. Thus, this is a (not nearly complete) model of this servo to allow you to create your model around it.
Also, here is the attachment wheel included with the motor: Datei:Micro-servo-9g-wheel.tar.bz2