Gregg, the Plugin Index thread (stickied in the Sigil Plugins subforum), will have a link that will take you to each plugin's dedicated thread/page. The very first post in a plugin's thread will have the latest version info and a download (or link to a download)for that latest version of said plugin.
Sigil Plugins Subforum
Sigil Plugin Index
Flightcrew Plugin Release page
The zip file with "unx" in the file-name is the one you want for Linux.