I like to keep installers for every version of macOS or Windows around in case I need them, and I know others around here do as well. If you’re missing the installers for Lion and Mountain Lion, you can now grab them directly from the Apple support pages. I discovered that I was missing an ISO for Lion, so I’m grabbing that now.
Lion: Mac OS X Lion Installer
Mountain Lion: Mac OS X Mountain Lion Installer
Source: Apple Makes OS X Lion and Mountain Lion Free to Download - MacRumors