Where can I download the version of Mac OSX I require.

Read the instructions her;

How to create a bootable installer for macOS – Apple Support

you will be able to download all OS's from El Capitan to macOS Big Sur

and then build a bootable USB flash drive for each.

This link leads to the App Store page for High Sierra. Clicking on the Open button results in a message: "This copy of the Install macOS High Sierra.app application is damaged and can't be used to install macOS".

This is also the same error message when I attempt to launch a copy of the full Installer (5.21GB) on my Recovery Disk. Was this installer always broken or has Apple crippled it? Has anyone else have this problem?

TIA.

Right you can see the App Store is saying Open, it should say Get or Download.

The error message is misleading the App Store has detected a copy of Install macOS High Sierra.app

somewhere on your mac, usually in the Applications folder.

Locate it, move it to the Trash, empty the Trash.

Connect your mac to your router via cable.

Restart your mac, go back to the App Store High Sierra download page

it will now say Get or Download.

Do not use the mac while it is downloading or any other devices that share the same internet connection.

Eau Rouge: Brilliant! Thank you, thank you! You nailed it. I forgot that I started the upgrade process last August by downloading. (I'm not sure why these files become 'stale-dated'.) I followed your instructions and everything went well. Yes, Apple does issue some confusing error messages. Thank you for your quick response.