Consumer Reports recently had an article on an older Prius. I think it was 10 years old, making it a 2002 I guess. Anyway, it had over 100K miles and the original battery was tested by CR and was practically good as new. Assuming you haven’t been in an accident, I think you may have just got a dud battery to begin with. The dealership is replacing it for free under warranty, so that’s about as good as they can do. They don’t make the battery, they just install it.
The earthquake in Japan is almost certainly the reason for the parts delay. It isn’t just affecting Prius batteries, it is affecting pretty much everything made in Japan. As said above, perhaps the dealership can let you drive one of their loaners while you wait for the battery. If not, ask them if they will offer you some compensation for a rental. Renting a car by the month can be pretty economical if you call around to the various places. Avoid the rentals from the airports I think. I’ve had best luck with long term car rentals from an Avis located at a nearby Sears retail store.
The only other thing I can think of is to call around to the local junkyards and see if they might have a used Prius battery.