Just to add a note here as nobody's yet mentioned it.
The various batteries for sale will be rated in mAh...milli-amp hours. This is the effective capacity of the battery and roughly determines how many shots you can take.
The various clones for sale have values from 750-2000 so are hugely different in their capacity....ie, 2000mAh models will take almost 3 times more shots than the 750mAh versions.
This is something to consider when deciding on price and value for money.
For what it's worth, I use Sterlingtek one's for both BP511 and NB-2L replacements and they have given no problems.
See the link in the traders forum...excellent value.
Bob