I'm not an owner, but I'd recommend checking out Walmart.
Usually you are talking about $80-90 for an excellent battery (Made by Johnson Controls) with a phenomenal warranty, and since Walmarts are everywhere....
I just picked one up for my 99 Boxster after my interstate failed within 2 years of purchase (it was my fathers and I don't feel like dealing with the warranty with his lack of receipt).
Edit: The Interstate battery failed because it was fully drained about 15 times due to electrical issues. I don't blame interstate.