The H-Mart in Arcadia has mangosteen sometimes. It is $10.99/ lbs. One bag is usually about 2 lbs. Worse than the price is the quality. 30% of the bag is rotten. I bought 4 bags and threw away a lot of it.
I ate a lot of them growing up in Asia, but according to my mon it is hard to find mangosteen now in the local market. Farmers converted their land into more economical fruit to grow, like grapes, or sell the land to the property builder.
There is no other fruit that really ressemble mangosteen, except some really exotic fruit from the deep forest in Indonesia, like Matoa, and some other I don't even know the names. But they are even harder to grow than mangosteen.