If you have access to a Mac, you can download the official installer directly from Apple. It will download as an application named "Install macOS High Sierra."

: A full High Sierra 10.13 ISO is available for public download. Creating Your Own ISO or Bootable USB

Apple has moved most legacy installers to a browser-based delivery system or direct App Store links.

Providing the to convert an .app file into a bootable ISO.

The safest way to get High Sierra is directly from Apple. Third-party sites often host "distros" that may contain malware or unauthorized modifications.

If you already have an earlier version of 10.13, download the macOS High Sierra 10.13.6 Combo Update to bring your system to the final stable version.

that downloads macOS installers and firmwares, bypassing App Store compatibility checks. macOS High Sierra Patcher