Obtaining nagios and Nagios plugins
The latest release of Nagios plugins can find out at -
https://www.nagios.org/downloads/nagios-plugins/
Also on,
https://www.nagios.org/projects/nagios-plugins/
Nagios core can be downloaded for free from below link -
https://www.nagios.org/downloads/
All previous releases are available on the official Nagios website.