I buillt my own. It was very easy and relatively cheap. I found some instructions at the AVS Forums or somewhere that I loosely followed, and it worked out great.
All I did was bought some pine boards, 4" wide (I wanted a thick border, though on a screen this large screen it doesnt look big at all) and 1" deep. I attached all 4 sides together using "L" Brackets and 3/4" screws. Then I bought some black velvet material and wraped the wood in that, using a staple gun to secure it.
For the screen material I used the "Blackout Material" that all these DIY'ers always talk about. It is the way to go; and they really will know what you're talking about when you ask for it at the cloth store. I attached that with staples as well, and I was done! Like I said, easy as pie.
I guess the only difficulty I had was in getting the screen material to strech smooth across the whole thing, but with a couple adjustments it was fine.