There are a number of places to find old commercials on YouTube. One is most likely to find old commercials uploaded to user's pages on YouTube, such as the user haikarate4 and Genius 7277. A basic search will yield many results.
Often a company will archive their old commercials on their own YouTube page. For example, Old Spice's YouTube page has the popular "The Man Your Man Could Smell Like" series of commercials. The user haikarate4 has commercials from several companies and decades posted on their YouTube page. The user Genius7277 also has old commercials from the 70's and 80's. Both users have the commercials set up in a variety of categories for easy browsing.