I know you already replaced, but, figured I'd toss in a bit myself...
Costco's Interstate branded batteries are apparently not anywhere near as reliable as their old Kirkland batteries. Yes, Johnson Controls are typically always the manufacturer of Interstate, but the specs and the warranty on the Costco Interstates - along with more recent track record with Interstates and a few mechanics in general hasn't been stellar. Usually, JCI manufactured batteries are fairly easy to identify, but honestly looked over several sizes at Costco just yesterday with their display, and couldn't find the branding on the plastic in any of the normal places or otherwise. Also, the Interstate Wikipedia entry is claiming there's deals between Interstate and Exide, now... though I can't find confirmation of such elsewhere. Personally, I suspect that they're cutting corners and cost by short-changing the CCA (cold crank amps) on the things, YMMV.
This said, best place for near anyone to buy a battery with a great price to boot? Hold onto your hats, it's Walmart. High turnover so the batteries are always fresh, Johnson Controls makes their high-end EverStart Maxx batteries (you can even find the JCI manufactured stamp in the sides), and Consumer Reports consistently rates them high. I used to just buy the best deal battery, but got stung with a lot of junk Exide batteries over the years dying after a couple three years of service, now I'm a Johnson Controls snob, and it's served me well. On years five and four with the last two batteries and still cranking. Stick with JCI or Deka manufactured batteries as Kevin mentioned, and you really can't go wrong. Given general availability and turnover, that means Walmart's now the most viable place.
tl;dr: Stay away from Exide batteries (which is most brands), and stick with JCI (Johnson Controls) or Deka/East Penn. Easiest place to find JCI is Walmart's Everstart Maxx (yellow label) line.
I just wish my own faith in Interstate (especially the Interstate/Costco) hadn't been shaken, but any supplier will sadly make to the specs they're given when money's on the table. Hopefully the past few years have just been a fluke.