I am setting up a puppet master and want to distribute nagios core 4 and nagios-plugins to each of my agents and the master itself.
Nagios core 4 is currently not available from the Ubuntu repositories so i used fpm to create debs from the tarballs and added them to the master sources.list by following these instructions : How to make my own local repository?
However, this does not propagate the debs to the puppet agents. How can I set up a repository to allow apt-get on the agents to use the debs?
There are many ways you could accomplish this. The question you linked to tells you a couple ways how to setup a local repository.
Once you have a local repository, you must configure APT to use it. this means updating the sources.list, or adding a file to sources.list.d with the information needed to connect to your repository. This will include information about the network protocol, and path information.
You certainly can use puppet to configure your apt sources. There are many ways to do that to.