Yo las he comprado en aliexpress. 7.4V 13000mAh Tablets Batteries DIY Cube U30GT, U30GT1, U30GT2 dual four-core tablet pc battery 33161125 Size:3.5 * 151 * 125 mm
http://www.aliexpress.com/item/7-4V-...874479827.html
(from AliExpress Android)
Ahora acabo de mirar y son 17,15$. Salu2