I can't help you with the United States but even in Australia they're not a common site in captivity, this only adds to their impressiveness. I can think of two collections that I've seen them in (Melbourne Zoo and Currumbin Sanctuary).
According to signage at the zoo, Miami is "the only park in the U.S. to display a pair of Black-necked Storks". That leaves the possibility of singles elsewhere or of birds not on show, unless it's just poorly worded, but they're certainly not numerous in the US.
I'd only seen them twice before - wild (and high-flying) in Nepal and at Zoo Negara, Kuala Lumpur.
