I checked on my own 10.8.5 system to make sure this works:
Download the macOS Mojave Patcher app from dosdude1.com - http://dosdude1.com/software.html
Open that patcher app, and click on the Tools menu, where you will find a link to download the current Mojave full installer app. That site downloads that app directly from Apple's server, so you should feel safe that it will be a good download.
That will be a 6GB download. Then you have two choices. You can run that downloaded installer, which will certainly upgrade your Mountain Lion system to Mojave. OR (better, I think) save that installer app to another drive for safekeeping, and make a USB bootable installer from that Mojave installer app. It's easy to find several different methods for making that bootable installer.
The Mojave upgrade WILL convert your hard drive to APFS, so I would suggest that you have a full backup before
you begin the upgrade.