Where is the Mac OS X Lion that I purchased? I bought Lion a year ago, but I downgraded to Snow Leopard because I wanted to play Starcraft, which is not supported on Lion. After Mountain Lion was released, I can no longer find Lion anymore. Does it mean that I can't get the Lion installer anymore?
Update OS X Snow Leopard to 10.6.6 or later such that it has a Mac App Store.
Open the "App Store" application
Connect with the same account (Apple ID) that you used to purchase Lion 
Click on the "Purchased" button on the top bar (picture 1, between Categories and Updates)
Find "OS X Lion" in the list (picture 2)
Click on the "Download" button


Et voilà!