Where can I purchase Mavericks?
There are two possibilities here as far as I'm aware:


Take the Mac to an Apple store where they may be willing to install Mavericks for you.
Find someone who is running Mavericks, or who has upgraded from Mavericks to Yosemite, and who is willing to help you. They will be able to download the Mavericks installer from their previous purchases in the App Store, and then they can pass the installer on to you via network or USB stick.