There is much to explore for the ephemerist in the Library of Congress Prints and Photographs online catalog.
For this series on theatre and entertainment (for Shakespeare week), I will confine this post to the 2114 performing arts posters. These include theatre, magic and minstrels and can be browsed as a list (with thumbnails), a gallery, a grid (as in the screen shot) or as a slide show or searched. Searching is aided by helpful drop-down lists: a search for opera, for example, prompts opera houses, opera bouffe, opera Normandy, etc as well as operas and operatic.
|(C) Library of Congress, Prints and Photographs Division|
There are hits for Shakespeare, of course, and additionally the Library of Congress has a special section in its Imagination Gallery (part of American treasures) devoted to Shakespeare in America, which includes ephemera.