OS X 10.11 is available to download on Apple store here.
Released: Sep 30, 2015
Size: 6.08 GB
How to create an USB installer for El Capitan?
1- You need to prepare an USB (8Gb at least), then use Disk Utility to format it into “Mac OS Extended (Journaled)”
2- To avoid any confusions, name the USB driver to ElCapInstaller
Run Terminal ( /Applications/Utilities/), copy and paste the following command line:
sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/ElCapInstaller --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app --nointeraction
Be noticed that "ElCapInstaller" is particular USB Driver name.
3- Hit the Enter key, then provide the password if required