Now that macOS Catalina has been released, it’s become more difficult to access the macOS Mojave installer for those who still need it. Fortunately, Mojave 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 Mojave page in the Mac App Store, available via the link below:
https://support.apple.com/HT210190
To access the macOS Mojave page directly, please click on the link below:
https://itunes.apple.com/us/app/macos-mojave/id1398502828?ls=1&mt=12
That link should open the MAS and take you to the macOS Mojave download page.
The Mojave installer itself will download via the Software Update pane in System Preferences.
In the event that you’re blocked from downloading macOS Mojave, 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: