You can get an inverter that will run the camera very cheaply, e.g. kragen (auto parts store) has 150w ones for $15.
If you don't already have an AC adapter (which are very expensive from canon, but can be found more cheaply on ebay etc), you might want to consider just making an external battery back that uses larger NIMH cells or something. In fact, a 7.2 volt RC car pack would probably be fine.
You could also get a DC->DC converter to run directly off of your car 12 volt, but this may well be more expensive than either of the above options.