I've used the popular unipolar 28BYJ-48 stepper motor in many designs. It's super cheap and easily purchased from Amazon or Ebay. Its torque is a little weak but can be improved almost 3X with a simple hack of converting it from unipolar to bipolar. The blog http://www.jangeox.be/2013/10/change-unipolar-28byj-48-to-bipolar.html provides more information on the implementation and advantages of this hack.
The hack requires prying up a plastic cover that protects the motor's PC board, cutting an etch, and then replacing the cover. Removing this etch allows the stepper motor to be used with bipolar drivers. Unfortunately, opening the cover frequently damages the cover and makes it difficult to re-attach to the motor.
This Thing eliminates the need to remove the plastic cover. It is a sleeve that slips over the back of the motor. It has a hole that aligns a 2.5mm drill to cut a hole into the cover directly over the etch that needs to be removed. I recommend cutting the hole in the motor's cover, removing the sleeve, and then use the hole in the cover to align the drill to then cut the etch. Only slight pressure on the drill is required to remove the etch. Verify that sufficient etch has been removed by testing the motor with a bipolar driver.