The community behind WordPress plugins is a huge one. There's not a central plugin-building company or anything. Developers all over the world create plugins and then distribute them across the internet. And, very often, they receive no direct compensation whatsoever. The best and the safest place to visit for WordPress plugins is the official directory at https://wordpress.org/plugins/:
There are nearly 50,000 different plugins available in the directory at the time of writing this book, and that number is constantly expanding with around 100+ new plugins being added every week. This is really impressive considering the fact that many of those plugins are very advanced pieces of web software and not just simple, one-script add-ons.
There's a search field available in the center of the page in the directory (visible in the preceding screenshot)...