You need the Mavericks installer. It's no longer available at the App Store, unless you have already purchased it.
If you have not previously purchased 10.9 when it was in the App Store, you will have to call Apple and ask for a download code for 10.9 so that you can purchase and download it.
I realize the 10.9 was a free download, but it still is called a purchase by the App Store.