I bought some at 99 Ranch in Daly City maybe a couple of months ago. Very expensive, you needed to buy a bag of 10-12 fruit, and some had gone bad. Not worth it. I have no idea when mangosteen season is, but I haven't seen them since.
4299 Rosewood Dr, Pleasanton, CA 94588
I've seen them in the Asian fruit/veg stores in Chinatown and at New May Wah in Inner Richmond. There used to be a seller at the Alemany farmer's market (selling CA grown mangosteens!) but I haven't seen them for at least several months (anybody have a report on what happened to the tropical fruit stand?)
New May Wah
719 Clement St, San Francisco, CA