https://www.cnet.com/news/find-streaming-tv-shows-and-movies-with-these-five-apps/

Yidio (download for iOS or Android) monitors major services like Netflix, Hulu and Amazon Prime as well as smaller services (more than 100 in total). When you find a TV show or movie, the app will list the streaming sites that have the show available. Choose one and you will be taken to their app or website, to buy, rent or watch it. Yidio also lets you watch some shows in-app.

Some will be free, like The Flash or Supernatural from the CW. If you're looking to only watch free shows, select Free TV Shows (or Free Movies). The downfall is that "free" can mean the streaming services offer a free trial that you have to sign up for to watch the show. Other services, like Tubi, will actually let you watch the show for free with ads.