Not all extensions are present in the BApp Store. In such a case, we need to download the extension files ourselves. To do this, use the Add button provided under the Burp Extensions section, and browse to the extension file:
In Extension type, we can choose Java, Python, or Ruby, and based on that, we need to browse to the actual extension file for the language we choose.
GitHub is a great place to find extensions. A simple text search will reveal different and interesting extensions being written by people all across the world: