For many years, at both FTM and our predecessor site, MacFixIt, we've made it a mantra that you should NEVER sell a Mac without including its install discs in the deal nor should you EVER buy one that doesn't come with them.
That was good advice then and still good advice
if you are buying older Macs. However, it should be noted that Macs manufactured since roughly mid 2012 do not ship with an install CD/DVD and in fact many those Macs do not even have an optical disc drive.
It looks to me like your only completely legal hope now is to get hold of a set of Snow Leopard discs.
Or a trip to the Genius Desk at the nearest Apple Store, tell them what has happened and ask that they install the latest compatible version of OS X/MacOS, which they will do, or at least the local Apple Store here will, even on Macs that have officially been designated as obsolete.