If you misplace your power supply and you're near a Radio Shack you can purchase one of their off the shelf supplies. The Mopho x4 Keyboard will operate on a DC voltage range from 13-15 volts DC. It needs a minimum of 500 mAh. Any power supply that provides a DC voltage between 13-15 volts and 500 mAh or more will work.
Here are the Radio Shack PSU specs:
Radio Shack Power Supply
Enercell™ 15V/1000mA AC Adapter
Model: 273-332 | Catalog #: 273-332
Enercell™ Adaptaplug™ M
Model: Adaptaplug M | Catalog #: 273-344
PSU plug specs: OD 5.5mm, ID 2.1mm, length 9.5mm