There are two types of plugins that are available for use with e107. There are core plugins and non-core plugins. The biggest difference between the two types is that core plugins are created by members of the development team and as such are packaged with e107. Non-core plugins are created by other developers who
found a need, developed the plugin, and then uploaded it for others to use. This is one of the greatest assets of open-source software.
The site for all e107 plugins is at https://www.e107.org. This is the official repository; however, I would caution you to go to the developer's website before downloading and installing non-core plugins, as often developers are not quick to post updates to the https://www.e107.org site.