I went online and typed in camera battery BP-511 and came up with a good assortment from Amazon, including Canon. I did purchase an extra battery for my Nikon....and found it to be very good.
I use the hahnel version of the canon BP-511A batteries for my 50D & have found them to be excellent, and much cheaper than the oem versions.
Likewise, on my old 400D I've got the energiser versions & they are great. No need to get canon originals for sure!
I recently upgraded to the Canon 7D and was asking a friend about getting a spare battery, as he has Nikon and does not buy Nikon batteries, and he said he find no difference, only in the price.
I have an off brand battery for my Nikon D300 (since I have the grip, I have two batteries in the camera at once, one Nikon brand, other off brand). Honestly... I can't say I've noticed any difference.
Likewise, on my old 400D I've got the energiser versions & they are great. No need to get canon originals for sure!