How can I download a OS X 10.11 installer?

If you had downloaded El Capitan as a first-time "buyer" then you can re-download it from your Purchases page in the App Store. If it came with your computer when it was new, then you can reinstall it using Network Restore. If you have an earlier version of OS X installed on the computer then you can download it as an upgrade.

Kappy wrote:

If you have an earlier version of OS X installed on the computer then you can download it as an upgrade.

If you have an earlier version of OS X, El Capitan is only available from the App Store if your machine does not support Sierra. If your machine supports Sierra then only Sierra and OSs which you have previously downloaded are available.

You can download 10.11 here: https://itunes.apple.com/us/app/os-x-el-capitan/id1147835434?mt=12

More info can be found here: https://support.apple.com/en-us/HT206886#requirements