In my experience most Broadway souvenir stands are open for all three! Sometimes the stand might change locations in the theater, but I know the souvenir stand in the downstairs lobby (where the restrooms are) is open before the show and at intermission. I would assume it is also open after the show as I believe it might be the only stand for this show. I don't remember seeing one anywhere else, anyway.
<-- Tevye, FIDDLER ON THE ROOF, March 2018