The easiest way to obtain the installer is through the macOS Catalina App Store page .

: Between 12.5GB and 18.5GB of free space depending on your current OS. How to Download macOS Catalina from the Mac App Store

If the App Store method fails or you are on a newer macOS version, you can use the Terminal to fetch the full installer directly from Apple's servers. macOS Catalina - Technical Specifications - Apple Support