When my battery died it seemed to go quickly - a few polite hints in the morning and then I was lucky to get the car started and get it to a shop that afternoon.
There is a NAPA service center near work. The owner is a Porsche buff, races, always has P cars in the lot, and sponsored our PCA autocross school, so I went there. They said, "we have a something or another in stock for a kabillion dollars, we can put it in right now and test your alternator" and I said "OK". I bought an expensive 7 year battery.
Lessons learned: I should have reacted at the first hint of a problem. There is a lot of good info and opinions on this forum I could have referenced. And I probably could have gotten a 4 year battery from Walmart for a third [or less] the price of the 7 year.
Oh well.
Bottom line, you don't have to use Porsche OEM.