If the library you want to use is not available via the library manager, you can get the source code and place it into your machine's local library folder. The location of the folder is rather specific. See the file hierarchy below:
On Linux machines, the folder is named "Sketchbook" and it is typically located in /home/<username>
On Windows and Macintosh machines, the default name of the folder is "Arduino" and is located in your Documents folder.
User installed libraries should go in a folder named Libraries, located inside your sketchbook folder. This is where the IDE will look for user installed libraries.