You have already mentioned the 1350Extended battery. This is supposed to be a 10-15% increase in charge and that can be had here for $44.95
Seidio Innocell 1350 Extended Battery for Palm Pre - PreCentral Store
There is a kit out by the same company that sells a 2600 extended battery, which requires you to replace the back plate on the Pre. It makes it a bit bulkier, but it claiming a 126% increase in charge. This can be had here for $64.95
Seidio Innocell 2600mAh Extended Battery for Palm Pre - PreCentral Store