There’s some power supplies suggested on the ‘Powering Motors’ page of the guide: https://learn.adafruit.com/adafruit-dc-and-stepper-motor-hat-for-raspberry-pi/powering-motors
The voltage used to control stepper motors is variable, and all controlled by the stepper driver. You just need to make sure the supply or battery pack you’re using is able to offer enough power, if you’re wanting to drive two motors from that HAT then use a 12v supply that can offer at least 5A.