50 Watts is an excellent place to find striking collections of design and illustration. From the about page:
50 Watts is a growing archive of weird and wonderful visual ephemera from around the world.
“Quite possibly the richest source of book-related design and illustration in the universe. Will displays the fervour of the most dedicated historian whilst time and again proving he has an eye for exceptional images.” —David Pearson
They also have a bookstore.