Once you buy the license you will receive a redemption code from Apple within 24 hours by email.Go to featured tab in App store on your Mac and select the link redeem on the right hand side under the Quick Links > enter the redemption code and then Apple ID and password, and it will allow you to download the Mountain Lion. Don’t worry, as you still have an option to buy the license for Mountain Lion as Apple still lets you buy license codes for Mountain Lion(OS X 10.8) , Lion ( OS X 10.7) and Snow Leopard ( OS X 10.6).
But if you have never upgraded your Mac or never downloaded any older version of OS X or Mountain Lion ( OS X 10.8), you can not get it anywhere.
As of writing Apple has a trend of completely removing the links of all the prior version of OS X from its website or App Store, leaving just the most recent one available to download, but if necessary you could request one from Apple by contacting them. If your Mac ran the previous version at any point in the past, you can re-download it from the Mac App Store’s Purchased tab.
