In addition to looking through sources such as magazines, newspapers, and television clips for advertisements, here are some good resources:
iSpot.TV -- click on Browse TV Ads and use left menu to choose a category.
AdForum's Creative Work webpage -- search for Brand, Country, Keyword, etc.
Advertising Self-Regulatory Council (ASRC)-- includes reports and case studies by the National Advertising Division (NAD), Children's Advertising Review Unit (CARU), National Advertising Review Board (NARB), Electronic Retailing Self-Regulation Program (ERSP), and the Accountability Program. For full-text access to some of the case studies, contact Research Librarian Mary Molineux at email@example.com or 757-221-3076.
AdViews A Digital Archive of Vintage Television Commercials -- part of the Internet Archive website. Thousands of television commercials.
Ad*Access -- images and database information for over 7,000 advertisements printed in U.S. and Canadian newspapers and magazines between 1911 and 1955