I don't think you can buy the housing anywhere as a stand alone product.
you can get a double-DIN monitor like the
DWW-6500, it's been out of stock digitalww.com for months but you can still get them on eBay. I'm not sure
how easy it would be to add you own computer components inside it because I've never seen one up close.
another option is to get a
Mekatroniks bezel kit and integrate your own screen into it. (I think they used to sell it in the MP3car store as well, but i don't see it there anymore).
good luck