The standard way to get euclid-wm is from the official release tarball.
If you are familiar with euclid and want a walk on the wild-side, you can test the latest development version, by checking out the git trunk.
euclid-wm is available from repositories for the following distros:
Note that these are graciously maintained by 3rd parties. I can't guarantee that they are up to date or fully functioning (some of them have fallen years behind the current release).
If your distro isn't listed, you should make it better by making a euclid-wm package for it or by convincing someone else to.
You can also browse the source online if you're into that sort of thing.
This site is optimized for w3m.