The Museum of Modern Art has a terrific store with all sorts of fascinating stuff. Visit their website at http://www.momastore.org/.
Also take a look at the wide variety of merchandise from the New York Public Library: http://www.thelibraryshop.org/. The library is also home to the original Winnie-the-Pooh.
Another great site is Washington D.C.'s Library of Congress. It's a little heavy on Americana, but they offer things that you won't find anywhere else.