If you have a compatible Mac, you can download the full installer from the App Store using the direct link provided in the How to download and install macOS - Apple Support guide. This downloads the Install macOS High Sierra.app to your Applications folder, which can then be converted into a DMG.
Before you download the 4.9GB file, make sure your Mac can actually run macOS High Sierra 10.13.6. macos high sierra 10.13 6 dmg