Q: Where can I download El Capitan?

Only on Macs that cannot run Sierra you are still able to download EC. It's not possible on more recent models (unless you already 'purchased' EC before).

Hello, may i suggest checking this out,

Download OS X El Capitan 10.11.5 Combo Update

You can still get it from the App Store using this link here

That is an update which requires the basic EC is already installed.

Here are the requirements for Sierra:

How to get macOS Sierra - Apple Support

And, IF your Mac does NOT meet the requirements, check the section under "Learn more" which contains a special link to get El Capitan - but that will only work if yours does not meet the requirements for Sierra.

I knew it would not work unless yours does not meet the requirements for Sierra.

For the past 2 - 3 OS versions, Apple has withdrawn the "old" version as soon as the new one was publicly available. So, you would have had to download El Capitan before it was removed. Make yourself a note to do that - you do not need to install it; just download the complete OS installer and park it in a safe place.

There wouldn't be any place that is "reputable" - why? Because the only place to obtain it legally is the app store. Anyone else offering it is offering a pirated copy because the license is tied to the Apple ID used to obtain it and is not transferable. That makes it illegal to offer it to anyone else.

Inscapes wrote:

 

so basically, it looks like Apple is forcing me to upgrade to Sierra, whether I want it or not...

No, Apple isn't forcing you to upgrade at all. If you wish to upgrade, your only option is macOS Sierra.

I just followed the link above to the app store and its letting me download it, I need to do a fresh install so will be using it to create a bootable USB as my MBP is too old for Sierra.