[PorscheDoc]

You can get a replacement battery at an autoparts store that will work just fine. Of all the brand name batteries in the United States, they come from 3 different manufacturers (One of them being Johnson Controls), which are then just labeled with the retailer's sticker. So your walmart battery and your die hard battery likely come from the same manufacturing plant. I buy all my batteries from Walmart, they are usually the cheapest, and my last one went 8 years before it died (and that was the cheapest black battery they carry).

edit: I would first try charging your battery and see if it holds. If the car sat a long time before you bought it, then sat for awhile after you bought it, it may have not had enough time to get a good charge in between.