How can I find "Install OS X Mountain Lion" (installESD.dmg) in the applications folder? I've just downloaded and installed the Mountain Lion update from the Mac App Store, but when I can't find it in my applications folder.

I want to copy installESD.dmg to be able to perform a clean installation.

How can I find "Install OS X Mountain Lion" (installESD.dmg) in the applications folder?
It gets removed once you perform the update. You will have to redownload it. It may tell you it's already downloaded, use the Option key and select the 'Downloaded' button and you can download again.