Looks like XMap for Joomla or XML Sitemap is no longer being supported and distributed any more. http://www.jooxmap.com/
We have a repository of it here for download. Their last version that was stable and usable.
Download Xmap Joomla XML Sitemaps
We may look at taking on the project and supporting it for our own clients. In the mean time, thank you to the devs at Jooxmap and thank you for allowing so many of us to use their extension for free for so many years.
Older versions of Xmap can also be downloaded from the JoomlaCode repository