Where can I download older versions of PSE?

You can download PSE 7, 8 (windows only), 9, 10, and 11 here:

https://helpx.adobe.com/download-install.html

You will need your serial number to license the download. If you registered PSE with Adobe, you can go to

http://adobe.com

and look for My Adobe or where it says, "Welcome, < your name >", and work your way through the links to your user account/purchases and you should see it there.