Q: Past Versions of OS - How Do I get One?

Prior versions of OS X are no longer available from the App Store.

You can try Amazon.

Any copy of Yosemite you may find on a site such as Amazon is tied to someone else's Apple ID. Go to a physical Apple Store and ask for it.

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Snow Leopard 10.6 DVD is still available for sale, but since say you are using 10.7 (Lion), chances are you do not want to go backwards.

Mountain Lion 10.8 is still available for sale ($20 US) from the on-line web based Apple Store.  You get a redemption code emailed to the account associated with your Apple ID.  You use the redemption code in the Applications -> App Store to redeem the code and download Mountain Lion.

Mavericks and Yosemite are no longer available for new downloads.  If you previously downloaded Mavericks or Yosemite, you can re-download them via Applications -> App Store -> Purchases, using the same Apple ID used for the previous download.

You may (or may not) be able to make a Genius Bar appointment as your local Apple Store and get them to install Mavericks or Yosemite.  That would be at the discretion of the Genius and the local Apple store policies (also how busy they are that day).

If you ever want the option to download El Capitan in the future, I strongly suggest you download a copy real-soon-now, that that it will be in your App Store -> Purchases tab for a future re-download.  You can "Quit" the El Capitan installer once it starts up, so you are NOT forced to install El Capitan when you download it.  Repeat this every year there is a new major OS X (macOS) release.

I want to download this version of the OS - How do I do this?

What version are you wanting?

If you downloaded the OS X before from the App Store, it will still be available in your Purchases

>AppStore>Purchased