To install OS X 10.11.6, your Mac must meet these minimum specifications: OS X El Capitan - Technical Specifications - Apple Support
You can obtain the installer directly from Apple through these official channels:
: If you already have an earlier version of El Capitan installed, you can download the 10.11.6 Combo Update (1.5 GB) to update your system directly. System Requirements
: While hidden from standard searches, you can still access the OS X El Capitan page on the Mac App Store via direct links.
: Apple provides a direct download link for the OS X El Capitan disk image (.dmg) . For the best results, use Safari to download this file.
