In 1935 President Roosevelt signed into law the Works Progress Administration (changed to Works Projects Administration in 1939) which spent billions to create jobs during the height of the Great Depression. Most of the money was spent on construction projects (many communities got a new library or post office or bridge) but a small part of it was spent on the arts. Artists and writers and musicians were put to work on projects that were to benefit the population at large.
One way artists were employed was in the creation of posters. From 1936 through the end of the program in 1943, over 2000 posters were created. They advertised theatrical productions, encouraged the use of public libraries, educated about the evils of syphilis, promoted our Nation Park system and promoted tourism.
The United States Library of Congress has a collection of 906 of these posters, and all of them are viewable online as part of the library’s Prints and Photographs Online Catalog. Even better, there is no known restriction on their use as they were commissioned by the US government as part of the New Deal.
Also check out the other resources in the Prints and Photographs Catalog. Many are pre-1923, and thus are in the public domain. Just be sure to check the rights and restrictions for each division. That information is easily found in the left sidebar.
Some of my favorites from the WPA collection: