If you've ever downloaded the El Capitan installer from the App Store to your iTunes account, go to the App Store app, and then to the Purchases tab. Find the OS X El Capitan item and click on the DOWNLOAD button. If it's greyed out and says "DOWNLOADED", it's on your system somewhere.
Do a search in Finder (⌥⌘ SpaceBar) and look for INSTALL. The file is called "Install IS X El Capitan.app".