The Public Domain
Every year, thousands of work have their copyright expire meaning they are now freely available to the public for free.
You may use such works and remix it to make it your own. Disney is a good example of a company that does this really well. For example, Frozen is based on the Snow Queen and similarly with all the other classics like Snow White, Cinderella, The Little Mermaid and Tarzan. They are all public domain works that Disney have made their own. Ironically, earlier version of Mickey Mouse have now fallen into the public domain. This follows Winnie The Pooh which went into the public domain last year.
If you would like to know more, there is a book by Professor James Boyle titled “The Public Domain: Enclosing the Commons of the Mind“
I would highly recommend you add your own twist to any public domain material you use. Platforms such as Amazon Direct Publishing expects heavy modification to public domain work before you can successfully self publish.
Some public domain sources that I frequently use are as below:
Books – https://www.gutenberg.org/
Movies – https://www.archive.org
Images – Rawpixel – Search for public domain images as they have quite a reasonable collection