Where to get an old version of photoshop?

Are you on a paid plan or trial plan? If its trial, then you can't install an older version. The trial is always for the latest version. Also, the link provided by Ali is for CC2018, but you seem to be searching CC2020 with a minor version rolled back. There are no direct download links for CC2020, so this will not work for you.

Now, if you are on a paid plan and still don't see any older versions, then I would suggest you see if there are any pending updates for the Creative Cloud application. If yes, then install them. If no, then try by uninstalling Creative Cloud using the Creative Cloud Cleaner Tool and then reinstalling it.

-Manan

You can roll back up to two major versions in the Creative Cloud app. You have to view Other Versions; see attached animation.

EDIT: As Manan points out in another reply, older versions might not be visible for a trial account.