Most likely all the retail stores will only have starting or dual purpose starting/storage but not true deep cycle. You can check for any golf cart service locations as they will have them. In a pinch, I have had luck with Bass, Cabelas, Battery Plus or other sporting /marine supply stores who had deep cycle trolling batteries.
If you're in a metro area, any place that sells batteries could order one for you and have them delivered to the store in a day or two. My local mechanic was able to get a 12v Lifeline Deep Cycle AGM delivered within 24 hours.