I've never seen really fresh (and good) mangosteens here in the US, although I have seen and bought sort-of-fresh ones at Chinese/Vietnamese/"Asian" (I hate that word)/International markets/groceries.** Whether some of them were brought in frozen then thawed out for sale I'm not sure. But they all have been uniformly bad, in line with what KennethT reports, especially as I certainly remember eating the fresh ones all through the time I grew up in SE Asia. (And getting my hands and clothes stained occasionally from squeezing-and-breaking the shell.)
** And they are some of the MOST EXPENSIVE things you can buy in these places...and for what, bad fruit...