While many third-party sites offer "cracked" or "direct" links, the safest method is always to go through Apple. Apple provides a direct download for El Capitan because it is often necessary to restore older machines. 1. The Official Apple Support Link
Apple often provides this as a .dmg file containing a .pkg installer. Mac Os X El Capitan 10.11.6 Installer Dmg Download
Getting Your Hands on the Mac OS X El Capitan 10.11.6 Installer DMG While many third-party sites offer "cracked" or "direct"