If you are looking for ads for billboards or whatever for your period layout, this site http://library.duke.edu/digitalcollections/adaccess/browse/ (http://library.duke.edu/digitalcollections/adaccess/browse/) has ads dating back to 1911. It covers everything from cosmetics to airlines.
Thanks for the link. I for one will be making good use of it.
Great link, thanks.
I have several vintage ATSF posters that came out of some of the old depots. I picked them up right berfore they tore them down. I am glad I have them now.
Redtail67
Some more:
http://www.signmuseum.org/
http://www.billboardsofthepast.com/index.html
http://www.roadrelics.com/
And, my own:
http://www.gtcom.net/~mahlkov/page14.html