Then take a look at the Vintage Ad Browser. With almost 125,000 ads from the 1800’s to the present, this resource was created by Philipp Lensen. Categories range from Animals (lots of pet food ads) to Tobacco to Alcohol, with a few very specific ones thrown in like Coke or Joe Camel.
The ads vary in scan quality as the collection was aggregated from a wide variety of sources. Links go all over the place as well and as a result, the metadata and information available with each selection is of varying quality.
However, it is an amazing feat to have amassed such a well organized display of vintage advertising on one site. As inspiration or a research starting point, I recommend the Vintage Ad Browser.