Yeah Library of Congress is great for this type of thing... I had to use it a while back for something and I got great search results... The copyright can be an issue though - seems like the LOC has no idea about most of the stuff on there.
Currently I am looking for a nice old image to make a life size cut out of... not turning out to be an easy task.
"(re)view is a collection of retro images that each tell a unique story by capturing moments from the past. From humorous, kitschy images to heartwarming, emotional portraits, the collection contains timeless subject matter such as the innocence of childhood, friends, family, business and industry."
There can be much more to the process that goes beyond the first meeting where you don't always get a budget and a contract on that particular day and you may have to put some money into the process before...