Wow, thats a tough one....
As an FYI, here is what they looked like.
http://www.old-computers.com/museum/...asp?st=1&c=409
Here are some keyboard cases from OKW, but a little too modern
http://www.okw.co.uk/products/okw/keyboard.htm
And here are some in aluminum
http://www.hammondmfg.com/dwg16.htm
Those are the closest I am aware of. You could probably dig through the lists of enclosure outfits on EITD.
http://www.eitd.com/ and see if anyone has such a thing.
I think it will be hard to find an old one to canabalize due to the collector nature of the TRS-80, but you never know. Someone might have a bunch of old parts. I'd be tempted to have one vacuum formed as it would be pretty cheap, but the TRS-80 case was injection molded, and thermo forming won't get you the nice profiles, and the secondary machining could be a pita. If you have a cad drawing, maybe a quote from protomold is in order, although I don't know if they can make them that big.
http://www.protomold.com/
Good luck
Ron