Does anybody know where can I download Catalina 10.15?

Click on the link below, use your Safari browser Firefox will not work.

How to upgrade to macOS Catalina – Apple Support

Go to Section 4 and click on Get macOS Catalina.

You will be directed to the App Store Catalina page

it should say Get or Download.

This will download the full installer which is 8.24GBS in size

and installs the latest version of Catalina which is 10.15.5.

If you really mean Catalina 10.15.0 then that is no longer available for download.

When first released the first version for download was 10.15.0, when 10.15.1 was

released Apple removes the previous version from their servers.

And so it goes on with 10.15.2 replacing 10.15.1, 10.15.3 replacing 10.15.2.

So if you have been trying to install an update to Catalina, then you can only

install that on a mac that is already running a version of Catalina.

Just download the full installer as described above.

See the information in How to upgrade to macOS Catalina - Apple Support

No I have just told you that Apple removes the previous versions of their full OS installers and only

keep the most recent version (at this point 10.15.5) on their servers.

Unless you find an unofficial site where you can download earlier full versions.