Depends on the show, but Broadway New York/One Shubert Alley/Theatre Circle have a lot. Posters more than t-shirts. Triton gallery has a lot of past posters too.
"This table, he is over one hundred years old. If I could, I would take an old gramophone needle and run it along the surface of the wood. To hear the music of the voices. All that was said." - Doug Wright, I Am My Own Wife