Just bought two bags of mangosteens in Chinatown for $20/bag (though she gave me a $2 discount on the 2nd bag). The bag is approx. 2 lbs, with 9 decent sized mangosteens inside. They look better than the ones I've found earlier in the season in the markets. Not the cheapest price, but I just tried one and it's good and juicy.
They're being sold by this 2 middle aged women on Beach St, right out in front of East Ocean City. They set up a table on the sidewalk, and work out of their car, selling a variety of fruits and veggies. Not sure if they're only there on weekends, but I recall seeing her there on past weekends as well.