system upgrade from Sierra to Mojave ONLY

Help! I have accounting software that will never be compatible with OS Catalina, so I do not want to upgrade to Catalina. However, my machine is running on Sierra 10.12.6 and I would like to upgrade to Mojave, 10.14.6. How can I do this? Everything I've tried sends me to the Itunes store, which is useless and doesn't have what I want.