Check out the open air markets in St.Marten and St.Thomas. I know you will find soursops and star apples. In St. Marten you can do a cab tour and ask to stop at the market in Marigot. You can also ask any others drivers and they will also know. They have a great selection of fruits. It is only open on certain days and I am not sure which ones. Otherwise there are many others that sell them.
Pay attention to what Jim said. You can't bring them home with you.
Also remember that you are buying "local" produce and your system isn't used to the water that was used to grow it and you could get ill from it. I personally haven't had a problem but others have.