I vote with Michael with the batteries he linked. OWC stand behind their stuff and are a trusted vendor to the Mac community.
Beware of batteries offered on auction sites. Too many are fakes, with Apple logos but not Apple quality. There are numerous reports in the Mac notebook forums here of such batteries:
1) being represented as new but show up with 20-40 cycles already on them already
2) failing to charge
3) damaging the computer
A friend has bought two G3 Powerbook batteries that worked great from this site:
http://www.global-batteries.com/index.php/cPath/1_12/sort/2a/page/1
However, when he needed a third, that vendor was out of the battery for his model Powerbook so he bought from another company. That battery does not work properly.