Afaik they aren't available for retail.
And tbh.
Why would you need one?
To protect your legit system?
Then you're better off with a Slim ;p
Protect a JTAG/RGH...
You can RGH Slims and late Phats so yeh it's not worth the trouble.
Besides you'll have to partially custom make them and their as expensive as a new 360, if not more expensive.
edit: They are available for retail which I didn't knew.
But still I think it's not really worth the money unless you insist on having some outrageous cooling mod.