I can't find Catalena on the app store.

I have a Mid 2010 MacPro with 4 drives and a Sapphire metal graphics card. Lots of my apps won't run on Catalena but I want to use it with some apps that won't run on 10.13. I plan to install Catalena on a spare drive to use when I need it. However, I can't find where to download Catalena. I searched for in on the App Store without success. Where can I get it? I have a copy of "Install macOS Mojave" that I downloaded so I wouldn't keep having to go through the Upgrade to Mojave routine each time I sat down to the computer. Should I install it and then update, or can I get Catalena directly?