Business Source Complete, Communication & Mass Media Complete, Academic Search Complete, and JSTOR are databases useful for finding articles about advertising, marketing, and branding. If you are interested particular techniques or channels, targeted demographics, or types of products, try search terms that represent them, in combination with more general terms like "advertising" or "marketing." You could try combining those general terms with the names of companies or specific products in order to find articles about specific advertising campaigns.
Mintel Reports provides profiles of the markets for various products and services. You can search for market profiles by keyword using the "I'm looking for box," or browse for profiles by hovering over "category."
These databases and electronic collections contain historical newspapers and magazines. They include the advertisements that appeared in those publications, which can be found by keyword searching for categories of products and businesses (e.g. drive-in theaters or gramophones) or specific products, firms, and establishments (e.g. Woolworth stores or Coca-Cola). Vogue Archive and New York Times (ProQuest Historical Newspapers) additionally have filters you can use to limit your searches to advertisements.