Can I buy Mavericks somewhere?

I don't believe that Apple sells Mavericks as a USB install unlike previous versions of OS X because it was the first free upgrade and therefore was never sold in stores. http://www.apple.com/shop/help/downloadable_software This link provides the older OS X versions to be purchased from Apple. I'm sure you can shop third party such as Amazon or eBay for a Mavericks USB install drive. Hope this helps!

Mavericks is no longer available to new downloads.

If you previously downloaded it during the year it was available, then it will still be accessible via your App Store -> Purchases tab using the same Apple ID. If you never downloaded it, it is not available to you.

You might be able to make a Genius Bar appointment at a local Apple Store and convince the people there to install Mavericks. You should have a good reason.

Otherwise, you can install El Capitan, or older versions if your Mac is compatible with the older releases.

If you want the future option to download El Capitan, I suggest you download it now, then "Quit" the install when it starts. That will make it available in your Purchases tab in the future. Repeat this procedure for each new Major macOS release in the future to keep your options open.

If you downloaded and installed Mavericks before, then it will be found in Applications -> App Store -> Purchases, as long as you login to the App Store using the same Apple ID you originally downloaded Mavericks with.

If you did not download Mavericks, but rather your Mac shipped with Mavericks, then you should be able to use Network Restore to get the operating system version it shipped with.

If you got Mavericks via some other method, then chances are you need to visit an Apple Store to get Mavericks.

For Your Information: Downgrading to and older version of OS X, requires you backup all your files, reformat the boot disk, and then install the older OS X release. Now you can use Applications -> Utilities -> Migration Assistant to restore your personal files from your backup. I strongly suggest at least 2 backups using 2 different backup utilities (eg. Time Machine, Carbon Copy Cloner, Superduper to name a few).

OS X El Capitan: Revert to a previous OS X version