My sympathy goes out to all who upgraded their OS X and have no means of going back to Mavericks. Except, of course, those whose units shipped with Mavericks.
If you obtained Mavericks through the App Store when it was available, you can always go back to it. Open the App Store, login to your account and click on the Purchases tab. Mavericks will be in your list.
It's funny how Snow Leopard, Lion and Mountain Lion are available for purchase at the Apple Store. Mavericks pulled from the shelf completely. I don't try to understand.
I don't think anyone gets that. Why have two even older versions of OS X still available, but not Mavericks? If a person says, "Well, because that one was free. They charge for Lion and Mountain Lion." Sounds plausible, until you realize Yosemite is free. So what difference would it make to leave Mavericks up? Both it and Yosemite would be free and it shouldn't make any difference to Apple which one a user prefers.