If you want to see a really awesome custom built case check out the links below, it's a case that my friend made. He's a furniture maker so he knows his way around the wood shop. Though I believe this is the first computer case he's made. Was built completely from scratch, the only things he took from a retail computer case was the cd drive/HDD trays and the rear IO shield and PCI slot covers. Even the motherboard tray was designed by him, I think it's plexi. Really a neat thing, his son also made a case with a 1940's wooden radio motif.
I'm also in the process of modifying an old full tower case that I have, putting a plexi window on the side, making mounts for water cooling system, and a double hard drive tray to fit in the cd rom bay. It's definitely not as elaborate as this scratch built case but it's functional. I was actually going to buy the Antec 900 case but it's so expensive and why even buy a new case every time you build a computer when you can just modify the one you have or just build a new one.
The computer tech I used to go to had an athlon xp rig with a micro board. His case for that was a mobo tray for the back, a piece of plexi for the top and side, the bottom was a piece of stained ply wood and he had two 80mm fans for front and back. I think the entire case was only as big as the micro ATX board and all the drives sat at the bottom. A case like that would cost maybe 20 bucks.
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If you just want a custom paint job I would seek out an air brush artist, or you could just do it yourself if you wanted just a solid color and no graphics. Heres a link to directrons computer paint section, I don't know if it'd be any good but it's a place to start.
http://www.directron.com/painting.html