Now that macOS Mojave has been released, it’s become more difficult to access the macOS High Sierra installer for those who still need it. Fortunately, High Sierra has not been removed from the MAS and it is still available for download. Apple has a KBase article that shows how to access the macOS High Sierra page in the Mac App Store, available via the link below:
https://support.apple.com/HT208969
To access the macOS High Sierra page directly, please click on the link below:
https://itunes.apple.com/us/app/macos-high-sierra/id1246284741?ls=1&mt=12
That link should open the MAS and take you to the macOS High Sierra download page.
In the event that you’re blocked from downloading macOS High Sierra, you should be able to download it in a virtual machine. I have a post on how to do this, available via the link below: