sterlingtek.com is a good source for 3rd party batteries. I have several for my 20D and they last a lot longer imo. That being said, the oem canon battery for the SD800IS I have currently lasts pretty long. Can't give you an estimate since I've only had it for about a week.
Are you talking about the SD700IS? The IS on these cameras are great

. I'd get the SD800IS for the wide angle, IS, and ISO 1600 (thought iso 1600 is very noisy). The digic 3 processor is also cool w/ it's face recognition and fast processing, lol. The bodies on these thing scratch and the paint rubs off so fast. In less than a week, stored in it's canon pouch, i've got rub marks around the frame of the LCD! Good thing it's not really mine

.