Non-Standard computer PCB, finding enclosure to mount
With limited information regarding the board and it's mounting hole pattern, it's hard to give specific information.

What I would recommend for enclosing general purpose large boards would be a 1U or 2U rackmount enclosure. They are fairly common for electronic equipment. If you lay your board inside it, you can mark the mounting holes with a sharpee and then drill the holes out. Once you have your custom hole pattern you can use some stand offs to secure it in place. You'll also need a few screws. You might consider using nylon standoffs if you want electrical isolation or the PCB mounting holes are connected to the ground plane. Below are a couple of links to get you started:

2U rackmount chasis (435mm x 295mm x 80mm): $39.95 http://www.circuitspecialists.com/rackmount-enclosure-37-2u.html

PCB heck standoff 1/2": http://www.digikey.com/product-detail/en/2203/36-2203-ND/82512