Below are a list of searchable/browsable web sites that feature examples of contemporary advertisements.
Google Images
Type in "[your company name or product name or brand name] and advertisement"
YouTube
Useful as a source for ads in video format.
Other web search engines recommended include: Bing Images, flickr, and Yahoo Images. Try keywords which include company, product or brand name, and the keyword advertisement or variant terms.
Proquest Historical Newspapers: The Globe and Mail (1844-2010)
Choose the advanced search option. Under Document Type, choose display ad or classified ad options. For further information, please consult this Proquest guide.
New York Times Historical (1851-2010)
The search interface defaults to advanced search mode. Note that under Document Type you can choose from a number of different ad type options, e.g., classified ad, display ad, and advertisement. For further information, please consult this Proquest guide.
Financial Times - Historical Archive 1888-2010
Apply the advertising limit available under "Limit Your Search."