Mangosteens carry the dreaded Med Fly and so are banned for import into the US.

There are some growers here in the US (florida and CA) also there are some in Hawaii (they cannot be shipped to the mainland because Hawaii is infested by the Med Fly) So if you are heading to HI you can get them pretty easily there I think. the domestic ones are very hard to find and very expensive. The frozen version are simply not worth buying. It is becoming easier to find the juice since it is getting a lot of attention from the supplement community for its antioxidant properties.

If you travel at all to Canada you can generally find the fruit there - I have gotten them in Toronto and Vancouver in the fruit and veg shops in Chinatown and in public markets. there is no problem importing them in Canada since the climate (at least until now) prevents the flys from surviving and creating an infestation. I have also found them in Sao Paulo.

I live in NYC and since there are periodic reports of mangosteens in Chinatown I always keep my eyes open for them - never seen them even in the "real" chinese markets. The good news is that you can find longans, rambutans etc so not all is lost on the delicious tropical fruit hunt.

there are some good articles in the NY Times archives, especially those written by RW "Johnnie" Apple for those of us who crave them.

PS - I have also bought them in London - have found them in fruit and veg on the Brompton Road, have aslo seen them in harrods and Fortnum and Mason - not really very affordable in those last 2 locations - even before the dollare tanked!