Does anybody know where I can buy a Computer Case that is 30cm deep or less (front to back)? 
The width or height is not a problem. This case will fit in a cooling CABINET. The computer will run Win XP Office ect. PLUS high end DTP & 3D Studio.I wouldn't put a computer in a cabinet of any sort, especially if it's to be used for high end work.
Computers can generate a lot of heat, they're much better in an open space.
That's a pretty small case too, especially for a high end system - you may find that powerful graphics cards will not fit very well in there.
If you're in the US, try Newegg - they have good prices on a lot of components, and you'll be able ti find a case that meets your SIZE requirements, although I do advise against it.Thanks for your ADVICE; I am aware about the cooling issue however I do require a case no deeper than 30cm front to back.  Most cases are about 40cm deep but I thought there may be a case out there that doesn’t conform to the usual format/layout.
There is no search engine that allows me to search on size alone. I have sent a request to various suppliers but I thought I would try this forum (you never know).
What you're probably looking for is a MicroATX or ATX Mini-tower case, which vary in size from about 8" - 18". But it you're doing high-end stuff, you're gonna want a nice video card, which most likely won't fit in one of these cases.

Here's some examples on Newegg.com:
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=2010090007+1054808292&name=MicroATX+Desktop
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=2010090007+1054808478&name=ATX+Mini+Tower

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