Where can I find the Mojave installer?

Use Safari:

https://support.apple.com/en-us/HT210190

Or via the Terminal (use the commands in bold):

softwareupdate --list-full-installers

Finding available software

Software Update found the following full installers:

* Title: macOS Monterey, Version: 12.0.1, Size: 12128428704K

* Title: macOS Big Sur, Version: 11.6.1, Size: 12428472512K

* Title: macOS Big Sur, Version: 11.6, Size: 12428553042K

* Title: macOS Big Sur, Version: 11.5.2, Size: 12440916552K

* Title: macOS Catalina, Version: 10.15.7, Size: 8248985973K

* Title: macOS Catalina, Version: 10.15.7, Size: 8248854894K

* Title: macOS Catalina, Version: 10.15.6, Size: 8248781171K

* Title: macOS Mojave, Version: 10.14.6, Size: 6038419486K

softwareupdate --fetch-full-installer --full-installer-version 10.14.6

The link to install MojaveOS on the App Store results in a message that "This copy of the install macOS Mojave application is damaged, and can't be used to install MacOS." Is there anywhere else to get the Mojave install file?

Delete any old "Install macOS Mojave..." installer from /Applications. Uninstall any "cleaners", virus apps, VPN etc malware. Reboot the router. Safe-boot and try again preferably via Ethernet or at least keep household's other Wi-Fi traffic to the minimum.

The installer is actually at the link that is in the Related Article above.

Be sure to use the Safari browser to download it, not Firefox or other browser as they may not work properly.

Yep.

Glad to know you sorted it out. 🙂👍🏽

Thanks. Deleting the old install application solved it. Strange that their error message just says the file is damaged instead of suggesting deleting old install files.