I don’t believe mavericks will be on option in the App store if not installed when available with an Apple ID. I have a machine here that is running Snow Leopard, and the only OS upgrade available in the App store is Yosemite until I log in with my Apple ID, and look under the purchased tab.
So a couple of things that might work. Call Apple support, tell them you need to update your OS but some software you need may have issues with Yosemite, so you would like to install Mavericks until those are resolved. They may provide you a way to access Mavericks through the App store. I had this very issue with one machine when upgrading and I needed to install Lion first.
If you have some friends that are mac Users that did install Mavericks, you may be able to hook an external hard drive up to their machine and have them install Mavericks on this. However, rather than installing it on the drive, go through the motions and after it has downloaded and before it installs, just copy the Install OS X Mavericks application from the Applications/Utilities folder to your hard drive. You may even have a friend who has a copy of it already. (I keep copies of all of these myself).
If you are near an Apple Store or certified Apple dealer, I believe you can take the mac in they can install older versions of the OS.
I suppose you could also download it from one of the many sites offering “pirated” software. technically illegal but since it was free maybe not so much of an issue, other than can you trust that it hasn’t been modified and contain viruses/key loggers/trojan horses.
All that being said, the main issues with Yosemite are WiFi issues, and some (like me) seeing some sporadic graphic issues. You may want to just clone your startup drive, and try Yosemite. If you are one of the few who see some wifi issues, just startup from your clone and re-clone it back. I personally haven’t seen anyone with the wifi issues, and the graphic issues haven’t forced me to downgrade my Mac Pro to Mavericks.