Downloading the OS X Mavericks Installer from the OS X Yosemite App Store

To be able to find the Mavericks installer app, you must have downloaded it at some point with the same Apple ID you are using to access the App Store. If you never previously downloaded it or installed it on a Mac, you will not find the installer available on that account.

Go to the  Apple menu and choose “App Store”

Click on the “Purchases” tab and log in to your Apple ID if you haven’t done so already
Scroll through the list of purchased / downloaded apps and find “OS X Mavericks”, then click the “Download” button next to its name