Everything is on the internet...
Most software for linux is free so...
No big store chain will carry them because they wont make money selling it
I not sure if I understand your question but
for the distros (Ubuntu, Fedora, OpenSuse, etc) you cant look at distrowatch.com
for the software search the internet or now the distro like ubuntu comes with Ubuntu Software Center witch make the installation very easy
PS. this is a Nagios Forum you will have better luck on a Linux Forum