Installing nagios3 on debian4 etch

[Kelly Jones <kelly.terry.jones@gmail.com>]

I want to install nagios3 on my debian4 etch system, but "apt-get
install nagios3" yields "Couldn't find package nagios3".

What should I add to /etc/apt/sources.list?

In general, how do I find which source provides a given package, and
what are good sources to add to sources.list?

I'm running a system of redundant VPSs, so can't upgrade to lenny yet
(not enough providers offer/support it).

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