Apple - Download El Capitan Installer now that Sierra is out

It depends on your circumstances. It can be difficult to find old OS downloads.

If you purchased it previously it will be in the App Store under the Purchased tab.

If your mac came with it pre-installed you may have to go the internet recovery or Recovery HD route, but that would entail wiping your hard drive in order to do the install. You could then migrate your user data from a Time Machine backup or similar.

Alternatively, if you have a bootable backup of your El Cap system (assuming you had it installed) you could reinstall that.

More specific answers will require more specific information from you (it's generally a good idea/helpful to provide more information so that answers can be geared towards your situation and others in the same situation can benefit).

Whatever you do, make sure you have a full working backup (which you have tested) so that you can revert should something fail.

The following link, opened in Safari, should trigger the App Store to show the El Capitan download even if it is not already in the purchases list.

https://itunes.apple.com/app/os-x-el-capitan/id1147835434?mt=12

I tried the link a few times but my App Store did not open. Maybe it was a server issue on Apples side or maybe it helped already having opened the App Store. Anyway today the link lead me correctly to the El Capitan download in the App Store.

Update: After that I tried the link a few more times by directly clicking or by copy paste it into the URL-bar. Sometimes it worked but sometimes not. So it seems a matter of retrying until success. Just reloading the page with Cmd+R did not have the same success rate as clicking into the URL-bar and resubmit the request by hitting Return.

Source of the link is the link of the OP

https://support.apple.com/en-us/HT206886

where it is provided under the words "get El Capitan from the App Store"

Update2: Here is what someone says on upgrading from Yosemite to El Capitan in an Apple discussion thread:

If you Mac is compatible with Sierra, then you cannot install El Capitan unless you had previously downloaded it. The version you tried to install is for those with Macs that cannot run Sierra or still have Snow Leopard installed. Since your Mac cannot run Snow Leopard either, there isn't a way for you to get El Capitan.

... too sad.

So one idea left would be to get an older Mac of a friend. Log in the App Store with your account. Download El Capitan. And finally have access to it via your purchases tab in your account from your newer Mac.

Since MacOS images are digitally signed by Apple, you can safely download El Capitan using eg. BitTorrent (link) or a reddit (link), as long as you

1. Don't run the Install OS X El Capitan.app file, but only write the OS image it contains (Contents/SharedSupport/InstallESD.dmg) to a USB stick (because, as far as I can gather, only the install image is signed)
2. Verify the digital signature over Contents/SharedSupport/InstallESD.dmg, inside the downloaded Install OS X El Capitan.app, as per this answer: https://apple.stackexchange.com/a/262914/93129