Where can you find JavaScript polyfills?

Yesterday, I gushed about why I love polyfills.

So, where do you find them?

1. The Mozilla Developer Network is a great place to learn about JavaScript APIs. They'll often include links to polyfills if they exist or are needed.
2. GitHub is filled with polyfills of varying quality. I typically look at any existing issues and the number of stars in evaluating whether or not they're any good.
3. Remy Sharp, who coined the term, maintains a list of his own polyfills. Literally anything he writes is awesome.
4. After a while, you may start to feel comfortable writing your own. I recently did that for the Validity State API after I was unable to find a polyfill elsewhere.