Although the distro for Mojave doesn't show up in the App Store you can still get it by using this direct link:
https://itunes.apple.com/us/app/macos-mojave/id1398502828?ls=1&mt=12
from there you can search YouTube for tutorials on how to partition a space on your drive to boot to Mojave. You switch back and forth by holding "ALT" during boot-up.