The pololu drivers are constant current. Look for stepper motors with a rated current of 1 to 1.5A per phase. The voltage at the rated current should be 4V or lower if you intend to use a 12V supply. If you are using larger than normal stepper motors and all the ones in that current range have a voltage drop of more than 4V at rated current, then drive the pololou drivers from 24V instead of 12V.
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