1) This battery (Apple High Capacity Battery for Apple iPhone 3GS)
Sold by i-mobile-accessories
=> It works, I have not had a problem with the installation.
2) Apple Iphone 3GS Battery Li-Polymer
Sold by SARL VENDOS85
=> Does not work (or at least, it is well recognized by the iPhone but does not load)
So personally, I recommend this farm.