Jennifer De Witt writes: "We are trying to locate collections of print
advertising from the mid-19th to the mid-20th centuries." (more stuff
deleted)
 
You might try the Archives Center at the National Museum of American
History (Smithsonian Institution). They have several collections
relating to advertising and an ongoing program to document the history
of advertising. I don't have an e-mail address, but their snail-mail
address is:
 
National Museum of American History
Archives Center
MRC 601
Washington, DC 20560
 
Paul Silbermann                  National Air and Space Museum
[log in to unmask]             Archives Division
****************** My own opinions, not NASM's ****************