Older, obsolete releases of Ubuntu are no longer available on localized mirrors. Instead, you must use the old-releases repositories. This is done by replacing the repository entries in /etc/apt/sources.list:

• country.archive.ubuntu.com → old-releases.ubuntu.com
• security.ubuntu.com → old-releases.ubuntu.com

For example:

us.archive.ubuntu.com/ubuntu/dists/natty/main/source/Sources

would become

old-releases.ubuntu.com/ubuntu/dists/natty/main/source/Sources