I've been using one of these for my Samsung B2710 handset and to power the portable stereo Bluetooth speaker system
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http://www.amazon.co.uk/TeckNet-Dual-Port-Universal-Incredible-Blackberry/dp/B000NDQ92WThe twin USB outputs are really handy, but you would need a Mains to USB PSU to charge the TeckNet Dual-Port iEP380 5000mAh 1.5Amp Output Universal USB Battery Pack.
http://www.amazon.com/New-Trent-iDual-Port-IMP50D-Blackberry/dp/B003690Q42The New Trent iDual-Port Pack IMP50D 5000mAh External Battery Pack available in the US is basically identical.