I've purchased the 3rd party batteries from
Home - MyDigitalDiscount.com Inc. - they seem to work well. They hold the same sort of charge as the factory batteries, and cost 1/5 what the Canon batteries do. However, in my experience, their useful lifetime is shorter - so they end up being maybe about 1/2 the price of the equivalent Canon battery in terms of useful lifetime per dollar spent.
But... at $10/ea or so, how can you beat 'em???