Straight from the Apple Support page:

How to create a bootable installer for macOS https://support.apple.com/en-us/HT201372

High Sierra Page https://support.apple.com/en-us/HT208969

High Sierra App Store Link (hidden from standard Search) https://itunes.apple.com/us/app/macos-high-sierra/id1246284741?ls=1&mt=12