Your local Mathews dealer should carry them. I have the Black Creek semi-ridgid case for my z7. The only place i could find it was through my local dealer if they dont carry it they should at least be able to order it. My case was about 175 bucks tho thats the only down-side.
I've got one that says Z7. It fit my old Z7. Don't know about the xtreme though. It's stashed away since I bought the Heli-M case for my Heli.