There are essentially three different ways to find TV commercials online. The first is searching streaming video sites, such as Youtube.com, the second through advertising and media design sites, and the third is via a company website.
You can find many recent commercials on sites such as Youtube.com or Dailymotion.com by searching for terms like "commercial" or "TV advertisement" and using the filter feature to limit your results to the most recent uploads. However, you are limited to what people and companies choose to share.
Your second option is to look for advertising and marketing industry websites, such as the Creative Work section of Adforum.com. If you're seeking a particular advertisement, you can seek out the company website, Facebook page, or Youtube channel, where savvy marketers will highlight their latest creations.