I bought this case
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1 for my Oasis 2. Since it doesn't work on the Oasis 3, I attached double sided tape to adhere it to the back of my Oasis 3. I have a skin on the back of the Oasis 3, so no issues damaging the back. The double sided tape works perfectly and the case & Oasis are solidly adhered to each other.
ETA - the sleep/wake feature does not work. While it would certainly be convenient if it did, it's hardly a showstopper. Just wake it up/put it sleep manually with the push of button.