Nagios Enterprises maintains repositories for RPM based Nagios XI installations and RPM and DEB packages for our cross platform monitoring agent, NCPA.
If you're running in an offline environment you have two choices for how to install Nagios XI. You can either clone the nagios and nagiosxi-deps repositories provided for CentOS and RHEL or download the offline tarball.
If you need help with these repositories, check out the Knowledgebase and Support Forum.