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:
For example:
us.archive.ubuntu.com/ubuntu/dists/natty/main/source/Sources
would become
old-releases.ubuntu.com/ubuntu/dists/natty/main/source/Sources
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