It will be in /Applications/Install macOS High Sierra.app
Apple does not officially host DMG files for older OS versions on their main downloads page anymore. They usually point you to the App Store, which downloads an installer app. However, for advanced users needing the specific DMG archive, we have archived the link below. Download Mac Os High Sierra 10.13.6 Dmg
[ BUTTON: Download macOS High Sierra 10.13.6 DMG ] (File Size: Approx 5.2 GB) It will be in /Applications/Install macOS High Sierra
Note: This is the official final release of High Sierra (Build 17G65). which downloads an installer app. However
If the App Store link fails because your macOS is too new, use this Terminal command (still downloads the .app from Apple):
softwareupdate --fetch-full-installer --full-installer-version 10.13.6