Thanks for the help, I got there eventually.
Just in case anyone else want to do the same, here are my steps:
I followed the link and downloaded: calibre-0.8.34.tar.xz, then used Zip7 to extract this to: calibre-0.8.34.tar.xz, then extracted it again which gave a folder structure. I didn't know what to do with the "grep" command, but in this extracted folder structure I found:
which appears to contain all the source code for the built-in plugins, including TXT2TXTZ.