One more thing.
Those who need an immediate or occasional solution might try an AA battery pack:
Link
For those who need confirmation, the product description of this Gomadic pack states it can be used either to recharge the internal battery or power the 350 when the internal battery isn't working:
Link
There are also official battery packs.
Haven't found an internal battery replacement for the PRS-950 yet, but here's a battery pack (I'll look for a better price later):
Link