Can't find it in purchases or in updates, so I can't install. Am I missing something obvious?
Go to the front page of the Store - the 'Featured" section. Download it. It downloads to your "Applications" folder. If the upgrade installation routine started, and you want an upgrade, fine, carry through with it.
If you want to clean install, cancel out of the upgrade, as the downloaded image is removed as part of the upgrade process. Anyway, after you cancel out of the upgrade if it started, you can create a USB stick installer or similar to do a clean install. A weird sounding Euro guy tells you how to do it here:
In the description section of the video on the YouTube website, he provides the terminal commands you need to create the bootable USB disc.