Ratings information is proprietary and very expensive, but large companies and publishers pay for access to the data so that they can publish bits of it in their articles. Search library databases for articles that include ratings and viewership information.
Some databases have official subjects for ratings information. Use a keyword search to find an article about ratings/viewership and then see which subject(s) are assigned to the article. Insert that subject into your search to find other articles with that subject.
Here are a few subject search examples: