I've only seen these wooden cases listed at the Grado franchise store 4OurEars. And if you actually try to order one, they say it is out of stock. And if you ask when it will be back, your answer is...
This is where I also saw the wood cases. When they first came out I believe they cost $100.00 but most had quality issue with their finish/construction if I remember correctly, so maybe Grado discontinued them?