Excel Suites cabins have the 1st option to purchase the cabanas. If there are any cabanas still available about 1 month prior to sailing they will open them up to non Excel cabins. Usually a banner offering the Loft 19 cabanas will appear half way down the page once you go to the website and open the manage my cruise tab
They also say you can purchase them by the day once on board, but I have never been on a cruise that showed any daily availability.