There is no official disk image for El Capitan, the installer can be downloaded as an app for the App Store. There is a disk image as part of the installer than can be used to boot the computer and install OS X. It is located in:
Code:
/Applications/Install OS X El Capitan.app/Contents/SharedSupport/InstallESD.dmg
But Apple says you should use System Image Utility in /System/Library/CoreServices/Applications to make images to deploy.
If you had ever downloaded El Capitan before, it'd be listed in the "Purchases" section of the App Store. Did you check there? If not, then I'd ask around to find someone you know who might have already downloaded it, and have them download a new copy.
The installer that Apple makes available for El Capitan at the links above won't even install on newer computers that can run Sierra. (It's a different build number than the one that works on all computers and has more restrictions set up in the installer.)