It's worth looking at 7dayshop.com they have both Genuine Canon Versions as well as pattern versions. Although there are plenty of other online shops / ebay where you can get either type.
I don't know how much you know about batteries so if you know this forgive me but when buying a pattern battery always look at the specification which consists of 2 things the voltage (e.g. 3.7V) and mAH rating e.g. 1500mAH. Always double check that that the votage is the same as the genuine article. The mAH rating basically tells you how long it will last. Never get on with a lower rating (e.g. 1400mAH) as you will be disapointed, the good news is that quite often you can find larger capacity batteries (e.g. 1650mAH) that last longer, upto a few hundred shots per charge.
Many of the cheapest batteries are significantly lower in mAH's than the genuine and you may only get a few shots off per charge. So watch out.