you can get 750mAh batterieson eBay for about $16 each. I've gotten a couple of them, and they work just fine.
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=3869758217
They say 3.6v on the side, but they're actually 7.2, and i've had no problems at all with the two i have. they seem absolutely interchangeable with the one that came with the camera, and the seller delivers promptly.