Over 7,000 U.S. and Canadian advertisements covering five product categories - Beauty and Hygiene, Radio, Television, Transportation, and World War II propaganda - dated between 1911 and 1955. A digital collection from the Duke University Libraries.
Not just ads, but you can find them if you add "source:life" to any Google image search and search only the LIFE photo archive. The "Search LIFE images" box at the site itself does not seem to work.
More than 11 million stock photography images. You can search by "advertisement" and your subject. For example, "advertisement train," or "advertisement japan* war."