News: use local and national news sources (see the "News Sources" tab) to identify specific races. You can then search for candidates names or locations on YouTube in order to view their advertisements.
MORE News Options: online or print articles in news sources may reference particular advertisements. They may link directly to or embed the ad within the news article itself. Here's an example from the Washington Post: https://www.washingtonpost.com/news/the-fix/wp/2016/10/14/republican-ads-are-starting-to-give-up-on-donald-trump/
Political parties: visit both local and national political party websites for their campaign ads. Don't forget the smaller parties, in addition to both the state and national arms of the Democratic and Republican parties. In addition to the various Tea Party organizations, there are other political parties, like the Green Party, Libertarian party, U.S. Taxpayers Party, etc.
Issues or particular races: look for special interest groups OR super PACs that focus on particular issues (again, you can use news articles to help identify these). They may sponsor their own advertising campaigns.
Stanford Political Communication Lab - campaign ads from 1994-2016
YouTube: search by candidate name (see above), by issue/topic, by the name of a special interest group, the region
New Republic's 2016 Campaign Ad Archive - includes ads from primaries and general election
Washington Post Campaign Ads 2016 - video playlist of 2016 campaign advertisements
HuffingtonPost: Track campaign ad related news from this popular liberal political opinion and news site. Includes links to some ads.
New York Times - The Caucus (blog) - political blog posts specifically discussing 2012 campaign ads. Often includes link to ad itself.
Washington Post's The Fix blog - Ads (2012 campaign)