How to install macOS Catalina

I have a 2019 iMac running Big Sur and have discovered that TurboCAD does not work with Big Sur. TurboCAD is not yet available in a version that runs on Big Sur, so I need to get back to Catalina. I want to install Catalina on an external drive just to use with TurboCAD and leave the rest of my system as it is. I have a formatted drive ready to go but can't find how to get the Catalina Installer. I have found and downloaded an installer but when I try to run it, I get the error message "this copy of the macOS Catalina installer application is too old to be opened on this version of macOS".