I was handed an EC2 instance running 12.10 (Quantal Quetzal), which had reached EOL. Due to some reasons, upgrading wasn’t an option. When I tried to apt-get update, apt snapped back at me with multiple 404s along the lines of:

W: Failed to fetch http://<blah blah blah>.ubuntu.com/ubuntu/dists/main/universe/binary-amd64/Packages  404  Not Found [IP: 80]

It turns out Ubuntu moves sources for EOL releases to a separate subdomain old-releases, so all I had to do was change the subdomain of all the sources in /etc/apt/sources.list, like so (note that you need sudo for it, and its also a good idea to backup your original sources.list file before making any changes):

deb http://old-releases.ubuntu.com/ubuntu/ quantal main
deb-src http://old-releases.ubuntu.com/ubuntu/ quantal main

Digital Ocean also hosts a mirror of these:

deb http://nyc2.mirrors.digitalocean.com/ubuntu-old/ quantal main
deb-src http://nyc2.mirrors.digitalocean.com/ubuntu-old/ quantal main

If you have a EOL Ubuntu distro that you need to maintain for any reason, you can head over to http://old-releases.ubuntu.com/ubuntu/dists to see if your distro is covered.