Think you'll struggle unfortunately.. Apple don't list / publish the out of date versions of their operating systems or make them available through the App Store, unless you've previously already downloaded them.
I've got access to every release since Mavericks.. if you're really stumped and don't get any luck anywhere else, I'd be happy to download it and write the installer to a USB pen drive for the cost of the pen drive / posting it to you?
*edit*
Did you see this page? https://support.apple.com/en-gb/HT206886