There are several avail see second link below. A more effective and less expensive solution is to leave your powered vent in place and add 20 W solar panel ( first link below) with a source selector switch. The 20 watt will easily run the fantastic vent at full power.
http://store.solar-electric.com/kyoceraks20.html
http://www.campingworld.com/shopping...an-system/4568