The Miriam and Ira D. Wallach Division of Art, Prints and Photographs: Print Collection, The New York Public Library. (1935). Policeman in front of New York Public Library Retrieved from http://digitalcollections.nypl.org/items/8fafbd87-31b8-9af0-e040-e00a18067062
If you type "political cartoons" into a Google search and click on the "images " link toward the top of the page, you will get subject categories from which you can choose.
Browse collections or search by genre. Here is the link for "political cartoons" genre. For a more targeted search, simply type search terms into the search box on the top right corner. Click on the cartoon to view. Each record also provides the correction citation in MLA, APA, Chicago Style, etc.