Sony is going to release a battery pack for the PlayStation Portable in December, it will be priced at $44.99:
The PSP Extended Life Battery Kit will work on both the old and new PSP-1000 and PSP-2000 models. This 2200mAh battery will offer 20% more battery life on the older models, and 80% more life on the new slim line. This battery will bulge out slightly on the slimmer PSP systems, so new battery covers will be included. However, those with Star Wars Ceramic White systems are out of luck: doesn't appear a new battery cover is being made for that particular model.