The American Popular Song Sheet Covers collection can be found at the New York Public Library. This collection came to the library from the estate of George Goodwin, a radio station director. The collection contains over 400,000 sheet music covers with 4,000 of them digitized and available online.
The collection is arranged so that the user can browse by year of publication or search by keyword. Once you click on an item of interest, you have the option to enlarge the image or print. You may click through each page of the sheet music to view before printing. There are also image details attached to each music cover that gives information, such as the composer, year of publication, and brief description of the condition.
Excellent recommendation — the online selection is a lot of fun to pore through.