Where one can find ancient Linux distribution installation images (ISO) such as RedHat Linux 5 (not RHEL), early versions of Mandrake Linux?
ftp://archive.download.redhat.com/pub/redhat/linux/ has every released version of RedHat. ftp://archive.debian.org/debian-archive/ has every released version of Debian. Both have sources, binaries and installation floppy images but not isos.
http://old-releases.ubuntu.com/releases/ has all out-of-support releases of Ubuntu.
http://snapshot.debian.org/ has almost every package that went through Debian unstable since March 2005.
ftp://distrib-coffee.ipsl.jussieu.fr/pub/linux/ has a grabbag of old-ish releases.
For some REALLY old distributions see oldlinux.org. Their collection is far away for complete but on the other hand there seems to be couple of gems like Caldera 1.0 prerelease or SuSE Linux 1.0.
Many educational ftp sites do still host quite ancient releases, but a quick peek lead me only to Red Hat 7.3 (at ftp.funet.fi), so that may still be a bit recent for you necromancer ... :-)
The FTP sites that host the files (like the .edu sites) usually don't delete the old files. Like here for example you can get manndrake all the way back to 2004.
Red Hat Commercial Linux 2.1 (from 1995) can be found at http://cd.textfiles.com/infomagic/imagicldr199511/disk2/ .
I have Mandrake 8.0 Traktopel on my website as of now, 6.1, 7.0, and 7.1 coming shortly; along with Caldera OpenLinux 2.2 and X11 and Console Utilities from TuCows Linux. I have only been able to get Traktopel working, and on the ancient VMware 3.x, so the others are untested.