I know that in the past if I've previously downloaded OS X El Capitan from the App Store I could download it again from the Purchased tab in the App Store, however now that macOS Sierra is out I'm not able to download it as it says INSTALLED (and grayed out) even on a system running OS X Mountain Lion 10.8.5. Pressing any key combination, e.g control, option, etc. doesn't change the button to DOWNLOAD. All other previous versions of OS X can still be downloaded from my Purchased tab in App Store as they still say DOWNLOAD and are not grayed out, thus active.
El Capitan Download Link Without App Store
Official OS X El Capitan Installer from the Mac App Store. Un-archive the zip archive and you will get the app Installer. A bootable ISO or bootable USB can be made from this installer. Instructions on how to do that are on the Internet/Youtube. Jul 18, 2016 Download OS X El Capitan 10.11.6 Combo Update. This update is recommended for all OS X El Capitan users. The OS X El Capitan v10.11.6 Update improves the stability, compatibility, and security of your Mac, and is recommended for all users. To apply the update, launch the Mac App Store on your computer and click the Purchased tab: OS X El Capitan 10.11.6 should be sitting there, waiting for you to download it. Now, launch the Mac App Store, go to the El Capitan page or Purchased tab and download OS X El Capitan from there. MAS will now download it from the local server at a high. OS X El Capitan 10.11 (Direct dmg download link) OS X Yosemite 10.10 (Direct download link) New versions of MacOS installers are typically available through the Mac App Store, accessed through a computer that is compatible with the latest Mac system software release.
El Capitan Download 10.11
The image below is from a system running OS X Mountain Lion 10.8.5, does not have OS X El Capitan installed on it or even the Install OS X El Capitan.app on its filesystem, yet it says it's installed and grayed out.