Cannot find OSX El Capitan installer after installation

You'll have to download it again because it's gone! The OS X Installer application bundle is automatically deleted when the install is finished, when it's executed from a running occurrence of the OS. In other words, when doing an in-place upgrade, one needs to first make a copy of the installer if needed to be used again.

As an example, when the Install OS X El Capitan.app was downloaded, it's saved in the Applications folder and a copy of it needs to be made, preferably to another location, first before proceeding with the upgrade.

Note that if you've been using Time Machine, there may be a copy of the Installer application bundle in the backup, however, timing is everything. That is to say, because of the size of the installer and the time of the last backup in conjunction with when the installer was downloaded and used to preform the upgrade, there is the possibility that the copy of the Installer application bundle is not a complete copy.