Although Ext JS provides great components, we usually will also want to develop our own components or maybe use other developers' components. The Ext JS community is great regarding this subject. A lot of developers share their own components, extensions, and plugins with the community. There are two main places you can find them:
Sencha Market: https://market.sencha.com/
Sencha Forum: http://www.sencha.com/forum/forumdisplay.php?82-Ext-User-Extensions-and-Plugins