Apple has started selling older versions of OS X through the Apple Online Store. Despite the fact that neither operating system can be purchased through the Mac App Store, you can still buy them for $19.99 each through Apple’s website. Rather than a physical disc with the software, you’ll get a code that can be redeemed on the App Store to begin downloading the OS immediately.
Offering Lion makes sense for older Macs since some Lion-capable machines cannot run Mountain Lion. Offering Mountain Lion, especially for $20, seems like an odd move since the entire line of Mountain Lion-compatible Macs can also run Mavericks for free.