One other option is to buy rechargeable batteries.
Quality AAA NiMH batteries and a
good charger can save a bunch in the long run. (I spent $50 on a Maha charger that only does AA and AAA batteries, but can do all sorts of wonderful and geeky things to prolong or revive batteries. My name's Jon, and I'm a rechargeable battery addict...)
or, like lt1 suggested, Radio Shaft and other electronics places sell car adapters that can be switched to different voltages. They usually come with a bunch of different connectors so they'll plug into almost anything.