My mac came with Mountain Lion. I upgraded to mavericks - and now I can't find mountain lion download
For some reason which remains unexplained, the App Store 'Hides' the Mountain Lion purchase once Mavericks has been downloaded. The Lion purchase remains visible, but Mountain Lion becomes 'Hidden.'

Fire up App Store and go to Purchases. Now go to the Store menu and select View my Account - enter your password when asked. Go to the 2nd Section iTunes in the Cloud and click on Manage - You'll find Mountain Lion in the list.