Once you have the .app installer, you can create a bootable DMG for backup or multiple installs:
Create a Bootable USB High Sierra Installer in Mac OS X 10.13 21 Jun 2018 — download+macos+high+sierra+10136+dmg+mac+install
Ensure your hardware can run the OS before proceeding. High Sierra requires at least 2GB of RAM and roughly 14.3GB of available storage Apple Support Supported Models : Late 2009 or newer MacBook Pro : Mid 2010 or newer MacBook Air : Late 2010 or newer : Late 2009 or newer : Mid 2010 or newer : Mid 2010 or newer. Apple Support macOS High Sierra 10.13.6 full installer - Apple Community Once you have the
. Version 10.13.6 served as the "final" stable build, offering the most polished experience, security patches, and support for Why Use a DMG File? DMG (Disk Image) Version 10
macOS High Sierra - Technical Specifications - Apple Support