MacBook OS X Mavericks reinstallation
For me at least, since I previously "purchased" Mavericks from the App Store, it showed up in my previous purchases list so I was able to download it. If you never actually installed Mavericks from the app store directly (i.e. upgraded an older Mac using the App Store) then it probably won't be there. This is a strange oversight on Apple's part - some apps out there still don't properly support Yosemite, and my workplace also has had problems with Yosemite working with their directory infrastructure, so we still use Mavericks as a matter of course.

If you didn't "buy" (read: download for free) Mavericks officially from the App store, your only option is to find a friend who did and ask them to create you a USB installation media. Instructions for doing this are easily found on the Internet.