I was able to download the macOS 10.14.6 installer from this page:Is there a secret Apple download link for Mojave now? I haven’t been tracking the Catalina release date, and never got around to downloading Mojave from the App Store, because I was waiting for it to be fully baked. Apple grudgingly provided a way to get Sierra after High Sierra came out, so I am hoping something similar exists for Mojave.
Step #4 will suggest installing Catalina, but the download link is after "If you still need macOS Mojave, use this App Store link: ..."
I was running this from Catalina, so after the download, it automatically launched and gave back an error saying “this software is too old” or something to that effect, which is not a bad thing. Quit the installer if you need to, and you will find the installer file in your /Applications folder, as usual.