Jolly Pumpkin. I believe they use the same barrels for all their beers, but contact and storage time vary. La Roja is probably one they keep in the barrels and condition longest, so will be the sourest, while others are on the shelves within a couple of months of brewing. Do some research on them, and buy bottles with the most recent bottling date (little sticker on the back of the bottle). The older the bottle, the more complex (and sour) the beer.
If you have any gallon jugs (can get them from LHBS, or when buying cheap wine of apple juice), put a gallon of your latest beer in it and pour in the dregs from a bottle of sour, add an airlock and leave for 6 to 12 months. They say you need to leave sours 12 to 18 months to really get the flavors, but I've found the JP bugs work pretty quickly, and have had noticeable sourness at 4 months.