I'm still running Mac OS 10.11 El Capitan. I've decided to upgrade, but I don't want to lose 32-bit support, so Catalina is not an option for me.
I found this official Apple page: https://support.apple.com/en-us/HT210190
However, the link on that page to download Mojave (https://itunes.apple.com/us/app/macos-mojave/id1398502828?ls=1&mt=12) doesn't work for me. (I have iTunes 188.8.131.52.) It only pops up a message from iTunes:
Item Not Available
The item you've requested is not currently available in the U.S. store.
How can I upgrade to Mojave?