It isn’t always easy to get all of the right pieces for a particular MacOS update/install.  For example, if you are running MacOS 10.13.5 already the traditional ways of going to the Map App Store or doing system updates won’t offer you MacOS 10.13.4.

Thanks to dedicated mac folks on the interwebs we now have a tool. See:

https://derflounder.wordpress.com/2018/02/27/using-installinstallmacos-py-to-download-macos-high-sierra-installers/

and

https://github.com/munki/macadmin-scripts/blob/master/installinstallmacos.py