Conversion kits are out there but are not commercially available yet, several companies are making them for the Prius specifically, one is in Concord Ontario, one in California and the EAA(Electric Auto Association) canverted a Prius at Maker Fair using lead acid batteries and a Micromanzinita battery charger.
Check out
http://www.eaaev.org/ and
http://www.pluginamerica.com/