The official way to get a "high quality" (authentic) version of macOS High Sierra 10.13.6 is to download it directly from Apple's servers . While Apple primarily distributes it as an installer via the Mac App Store, you can use tools like
Because High Sierra is no longer signed by Apple for direct download (Apple removed it from the App Store in late 2019), many third-party sites host DMGs. To ensure , verify: macos high sierra 10136 dmg high quality