Try contacting antiquarian bookseller organizations for a list of members that sell prints and posters and start making phone calls. There are a lot of these stores with piles of old posters, prints, and maps in their stores that they don’t list online.

In the US check with the Antiquarian Booksellers Association of America (abaa.org). You’ll also find examples via Bookfinder.com and other meta book search sites.

Paul

Oak Knoll Books always has a few. http://www.oakknoll.com/