External power supply necessary to run two stepper motors from a RPi?
I'm not familiar with the raspberry-pi, but there's a good chance the stepper motors draw more current than the GPIO pins can supply. It's better to supply these motors with their own power supply. 

Depending on what voltage your motors require, you can use AA batteries. If the motors require lots of current, you may be better of using a LiPo battery of the appropriate voltage instead.