Installing OS X not from App Store This question might have been answered somewhere but have not been able to find it. So, I have a Mid-2009 MacBook Pro where I am still running OS X Yosemite since I forgot to update to OS X El Capitan when it was on the App Store before they changed it to macOS Sierra.

If I try to install macOS Sierra through the App Store it doesn't let me because it says that:

"This version of macOS 10.12 cannot be installed on this computer."  

I started looking to see if I can find OS X El Capitan but there is no result through the Mac App Store nor Apple Developers site. So my last resort was through Google, where I found several options on where to download OS X El Capitan version 10.11.

So, my main question is if I download from these third party websites, can I install on top of my current OS like it is done through the App Store or do I have to do a clean install?

Would appreciate any help.
You have installed Yosemite before, look in the AppStore whether Yosemite is in your purchased items. This is one Option to install Yosemite and upgrade to El Capitan.

You can download El Capitan with this direct link from App Store. But I'm not sure whether this link work for you, try it. 
Cannot offer other download links because many sites where you can download this are not clear trusted sites.

Third Option, went to next appstore or similar Mac shop and get a copy of El Capitan if possible. You have also the option to contact the Apple Support.