I picked up a Fuel+ at Fry’s Electronics to power the Core. It was $15, And the awesome color of white matches the Core. 
So Happy.
http://www.frys.com/product/7833999?site=sr:SEARCH:MAIN_RSLT_PG
2200 mAh
recharges in about 2 hours with USB power adapter, longer on a computer USB port.
Thermal & Short shutdown
Low Voltage cutout protection