UPDATE: There is some news of late about fresh mangosteens being available in some Whole Foods markets in California.
The mangosteen fruit is flavorful and juicy but also very delicate. In other words, it does not have long shelf life. When you are shopping around, try to avoid mangosteens with discoloration on the rind.
Not long after its harvest, the rind is losing its moisture, gets dark and is hard to cut through. If white spots start appearing on the dark purple surface, it means the rind is now sucking moisture from the fruit inside. At this point, the fruit is past its prime. Refrigeration slows this process down a bit.
The best and easiest way to get your hands on this “unicorn of fruits” is by ordering it online.