Try an H Mart in Fort Lee, NJ, or the nearby areas (in NJ).
You should be able to find the country of origin on the label of the bag or on the sign at the supermarket.
@liuzhou: with all due respect, if you're in China right now, as your location status indicates, it seems like you'd have a better time getting fresh mangosteens than in @cdh's location, Philadelphia.
I recall buying these fresh in either Vietnam or Singapore (memory is a little hazy at the moment), and it wasn't terribly difficult to eat them.
If you search for "mangosteen in us," there are some interesting results.
https://www.nationalgeographic.com/people-and-culture/food/the-plate/2016/05/meet-the-mangosteen/
The article above references mangosteens cultivated in Puerto Rico, and difficulties growing them closer to the US.
https://www.freshplaza.com/article/2181747/us-demand-for-mangosteen-strengthens/
The piece above talks about mangosteens being grown in Mexico and Guatemala.
https://www.thekitchn.com/mangosteens-52332
This says the import ban on mangosteens was lifted in 2007. Could be my hazy memory again, but that seems to coincide when Kesar and Alphonso mangoes were allowed to be imported into the US.