The official website of Redmine has a plugin directory, which you can use to find a plugin that you need (the directory, by the way, is implemented with another plugin that was written by Jean-Philippe Lang). However, many plugins on this list are not updated. For example, a plugin might be listed for an older Redmine, while its more recent version can actually be available. Also, more recent versions of some plugins can be available elsewhere, for example, on GitHub. (Sometimes, plugin authors, including me, fail to release updates for new versions of Redmine on time. So, volunteers can fork and update such plugins on GitHub.) Therefore, unfortunately, it is usually not enough to use the official directory to find a plugin for your version of Redmine.
Tip
A forum dedicated to plugins
In addition to the plugin directory, http://www.redmine.org/ has a forum that is dedicated to plugins. On it, you can discuss plugins, ask for help, request the development of a custom plugin,...