You are going to want a set of AA Ni-MH batteries. They have ratings anywhere from 1700MaH to 2300. They run about 10-15$. They are rechargable as well.
Should last you at least 150 upto 200 pictures per charge with your Canon.
If you don't have a charger, Digipower has a pretty good set for a good price. It's 30$ and you get 4 AA Ni-MH 2300MaH rechargable batteries with a charger than connects to the wall AC or a car's DC power. They look like this. I know my Compusa carries them.