how can I download el capitan

Source another mac in order to download the El Capitan disk image,

this mac must be capable of running El Capitan.

Click on the link below,

How to get old versions of macOS – Apple Support

Go to Download OS and click on OS X El Capitan 10.11

this downloads InstallMacOSX.dmg to your Downloads folder.

The next section can only be done on a mac that is capable of running El Capitan.

This includes macs that have the potential to run El Capitan but have been upgraded to a newer OS.

A mac that came preinstalled with an OS later than El Capitan will refuse to do the next bit.

When downloaded open to InstallMacOSX.pkg, double-click on

that and an installation window will open, this does not install El Capitan

but converts the InstallMacOSX.pkg to the Install OS X El Capitan.app which 

you will find in your Applications folder, it should be 6.2GBs in size.

(If the installation window asks which disk you want to install to, you must pick 

the disk that you are booted to at the time. Not any internal or external disk that 

you want to eventually install El Capitan on, that is for later.)

Create a bootable USB installer disk using the Install OS X El Capitan.app in the Applications 

folder and the createinstallmedia command in the Terminal app. 

Read the instructions here,

How to create a bootable installer for macOS – Apple Support

jfb00 Said:

"how can I download el capitan: our old laptop crash, i installed new hd. I am trying to install the osx using usb but need the el capital download. How can I download? Thanks"

Use a Different Mac:

Go Here: How to Create a Bootable Installer for macOS - Apple Support. Have access to another Mac? Ask someone else (i.e. family member, friend, co-worker, peer, roommate, etc.) if you may use their Mac to create one, with their permission of course.