How can I upgrade to any macOS version that is no longer available in the Apple app store? I am using a Mac Mini (Late 2012) that is running macOS 10.9.5 (Mavericks). I avoided upgrading to macOS 10.10 (Yosemite) when it was first released because of all of the stability issues surrounding it and have stuck to macOS 10.9.5. Never “purchased” or even attempted to download it via the Apple app store.

Now that macOS 10.11 (El Capitan) has been released, what can I do to upgrade to macOS 10.10.5 if/when I wish to? When I am in the Apple app store the only option I see macOS 10.11 as an upgrade option; screenshot below.

The logic is, I assume that macOS 10.10.5 is more stable that macOS 10.10 when it was first released, so I would like to put a 10.10.5 upgrade on my plate of options if/when I have the time and patience to deal with. I would like to avoid macOS 10.11 (El Capitan) for a while if possible.

Is my only option at this point to download a macOS 10.10.5 installer from some non-Apple website somewhere or does Apple have any accommodations to allow end-users to download macOS 10.10.5 from them in some way?

I see this OS X Yosemite 10.10.5 Update as well as this OS X Yosemite 10.10.5 Combo Update is available for download, but would any/either of those be useful to someone like me running macOS 10.9.5 in any way? I assume those updates are only useful for someone already running 10.10 already, correct?
As soon as a new Mac OS is released, the previous one becomes unavailable.

The only way to get an older OS directly from Apple is to have previously 'purchased it' - even if it was free - then it will appear in your purchase history in the App Store. 

If it's not in your purchase history, then you'll have to get it from a trustworthy friend [or less-trustworthy torrent etc]  

10.10.anything will do, as you can still get the combo updater to 10.10.5

It's always wise to get a new OS download, even if you don't intend to install it right now - just so it's always in your history.  I have every OS back to Lion in my purchase history - good for if I ever need a disposable OS for testing etc.



As Sierra will not run on older machines, this rule has been relaxed for machines whose last supported OS is El Capitan.
You can download it from here - https://itunes.apple.com/app/os-x-el-capitan/id1147835434?mt=12 - but it will not run if your machine is capable of running Sierra.
If it is capable, then you are back to needing it in your purchase history already.