Look on YouTube.com. Look for the official sponsor or company's YouTube channel first. If there isn't one, or you still can't find the ad, open the search up to everyone on YouTube who many have recorded the TV ad and uploaded it. Be aware that the quality may not be as crisp as in the original version.
Contact the company that's advertising the product in the TV ad. The company may have copies on its website, or can direct you to the advertising agency that produced the ad. You can usually find the phone number and website on the product's label.
Google the TV ad and look for search results pertaining to the advertising agency or video editor who worked on the ad. Most agencies and video editors showcase online reels of their TV commercials to attract new customers and generate buzz. The reel will also offer the highest quality footage available and is often downloadable to the public. To get started, find a list of advertising agencies at NewYork411.com.
Visit the online center for the Motion Picture & Television Reading Room at the Library of Congress (Loc.gov). Its librarians can assist with research requests. Be advised that you can't physically make copies of the ads you locate. Once you find the ad, you can research the advertising agency that worked on it and request a copy.
Visit the online archives at America TV AD History & Showcase (USATVads.net) to find commercials dating back to 1975. The organization charges a per-visit fee to browse the site and an additional fee to copy the materials. While contacting the advertising agency of the commercial is the easiest way to obtain a free copy, the America TV AD History & Showcase is a great resource if you've come up empty-handed in previous searches.