Apple no longer promotes High Sierra in the App Store, but they host a direct support link:
hdiutil create -o /tmp/HighSierra -size 8000m -layout SPUD -fs HFS+J Use code with caution. Mount the new blank image: macos high sierra 10136 dmg
If you have access to a compatible Mac, the safest way to download High Sierra is directly from Apple. Apple no longer promotes High Sierra in the