Where can I find "Install Mac OS X Lion" in the App Store?
What you're probably looking for is a copy of the OS X Lion installer. If you saved a copy when you first installed Lion, great. If not, you'll need to re-download it from the Mac App Store.


Launch the App Store and &#x2325;-click the Purchases tab. An "Install" button should appear next to Lion. Click it to re-download the installer.
Once the installer finishes downloading, go to your Applications folder to find the installer itself. It is called Install Mac OS X Lion.


Further steps to get the DMG image:


Right-click the installer and choose Show Package Contents from the contextual pop-up. 
Navigate to .../Contents/SharedSupport. There you'll find a disk image called InstallESD.dmg.