If you ever did the OS install version you want or need via the App Store, if you log into the App Store with your Apple ID you should find the one you want in the Purchased
tab. If so, download it, store it somewhere for safekeeping and then use it for whatever install you want to do.
Another option, did you have a bootable clone with the Mac OS version you want? If so you could use that clone (or TM backup) and clone it back using CCC (Carbon Copy Cloner) or TM or Recovery HD boot and Disk Utility, and then create a "tester" admin account and then delete your user folder and everything else non-apple you might want to remove.
Otherwise do some Googling for other various methods you could possibly use that Apple really FUBAR'd big time.
These are just some very condensed suggestions.
PS: I'm sorry for the delay getting back to you and I'm also a bit surprised nobody else has bothered answering.
You might get better Mac help if you want any, by joining up with Mac-forums.com:
Some good gurus there and some old ehmac members!!!