I got my spare 40D ones from a seller on Ebay
from this seller here
Honestly you get them to your door quick and they are 1600 mAh .
I can not flatten mine they last ages and ages.
And they are only £3.20 for the pair + the postage from Hong Kong which is £6.00 to the UK.
Do not be put off by the cheap price. I am proof that these are spot on.
I use them in pairs in my grip.
I hope this helps
Steve
Both Canon originals or compatibles.
Try 7dayshop. They do a pair for £9.49 plus £3.95 P&P from the Channel Islands.
http://www.7dayshop.com/catalog/product_info.php?cPath=777_3&products_id=100915
I've had a couple for 20 months and both are still working well.