An electronics place like Radio Shack should have a "voltage doubler" available that will support a USB charger. The current draw is super low for most USB devices, so there's not much risk of overloading the doubler by demanding too much current from it. A "6V" charging system actually operates at 7.5V against a fully charged battery, so the output of a doubler will be 15V.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!