So, I thought it might be good to finally write down where to find non-recent Ubuntu repositories.
Once, a distro's repositories are removed from official main mirrors, they are moved to "archive.ubuntu.com":
1) Stage 1: old
After some time, they are removed from "archive.ubuntu.com", too. Then the last resort where to find the old repos (and installers!) is on "old-releases.ubuntu.com":
2) Stage 2: really old