If you need for compatibility with older 32-bit apps or legacy hardware, here is the official way to get it from Apple.
If the link doesn't work, search the Mac App Store for "macOS Catalina" – it should still appear as "Get". macos catalina 10.15.7 download
Click this link on your Mac: 👉 macappstores://apps.apple.com/us/app/macos-catalina/id1466841314?mt=12 If you need for compatibility with older 32-bit