To properly power the stepper motor, you need at least 12V. A USB battery pack typically is only 5V and will not provide enough juice to push the weight of a camera (be it a lightweight GoPro or Mobile Phone). I highly suggest using either a 12V power wall adapter or 12V battery pack.
I mostly use the camera slider indoors. Having it plugged into the wall is nice because I don't have to worry about batteries getting low and dying. This 12V DC 1000mA power adapter works great with this project.
If you plan to use the slider outdoors, I suggest using an 8 x AA battery power pack. This features an on/off switch and 2.1mm barrel jack that plugs directly into the Adafruit Metro.