I don't know what kind of volume you are looking for, look on ebay U.S. (possibly others) for juice at least. I would think your best bet would be to find a large Asian market near you, they may carry the juice and possibly the fruit.
I don't think that Europe has the super strict import regulation on mangosteen fruit that the US does (don't quote me on that).
Some of the juice that is out there is heavily multi-level marketed (a rip-off), they are more interested in selling distributor licenses.