[Q] Best site to buy a battery from. Pad & dock topic

Hey, so I just picked up an Asus TF101 at a killer deal from someone and I noticed the tablet battery itself only lasts 2 hours, I haven't charged the dock fully yet but doing that tonight, but I'm guessing it's going to run pretty low as well.
I am planning a trip to somewhere is minimum power outlets and will need to put in new batteries. Anyone have any recommendations for sites where I can pick up some cheap batteries? I live in Canada, but I might be going to the states soon so I can easily get it shipped there.
Thanks!