For my digital collection I'm scanning in some of the sensational posters of yesteryear from the early Christies sales, organized by Bruce and Company.
You can buy these catalogs here for $40:
http://www.emovieposter.com/books/books.php?section=catalogThe posters themselves (and all images thereof), of course, have been in the public domain for several decades. (In fact most of them immediately fell into the public domain when they were first distributed/published because they were not properly copyrighted).
Many of these posters never reappeared at auction and, consequently, no good images of them can be found on the Internet.
Devil Is A Woman, The (1935 US 1S):
Frankenstein (1935 Spanish 1S):
Devil Dogs of War (1935 US 3S):
Phantom of the Opera (1925 US 1S):
Another Phantom 1S: