The English name for the Annona muricata is soursop fruit. However it is most popular in Central and South America, where it goes by the Spanish name guanabana and the Brazilian Portuguese name graviola.
How much antioxidants the soursop has is more than some other tropical fruits like pineapple (562) and banana (795), though it's low relative to a lot of what you find growing in colder climates. For example, your average apple with skin (3,049) is about double the amount. A lot of superfoods have ORAC values which are dozens of times higher than guanabana, some even more than 100x higher.
Where can you buy soursop fruit in the USA? Finding it fresh will be a challenge. Your typical grocery stores like Kroger and Meijer won't have it, nor do you have much of a chance of finding it at Whole Foods. The one exception is in Florida. In season, you can often find it at Latin-oriented markets and sometimes even a chain like Publix will carry it.
Elsewhere in the United States, you will sometimes find soursop fruit for sale at Asian supermarkets. For example in Southern California, you can find it in Westminster in the Little Saigon neighborhood, where you will also find mangosteen, cherimoya, and similar exotic tropical fruits. To a lesser degree, some Latin markets will also stock it.
Research Support, Non-U.S. Gov't: Arabian Journal of Chemistry ScienceDirect Jan 2016