I don't think it makes sense to get a (spare part) chassis from HP for that mainboard. I guess you won't find any used one as only chassis parts may be sold used on Ebay/Amazon etc. It's too exotic to look for an empty chassis. New one might be far too expensive, I guess.
From my research and above link, Suman provided, this board was only built into racked server/storage systems. So there will be no stand-alone Tower chassis for it from HP.
Can you provide the dimensions of the mainboard (could not find them, yet)? It looks to me that this is E-ATX (extended ATX or also called SSI EEB), but might be even HP proprietary. You may find a fitting chassis from e.g. Chieftec, take a look at UNI/MESH/BRAVO series chassis, e.g. BH-02B-B-B or as a full tower BA-02B-B-B
There might be other options from other chassis manufacturers, too (Lian Li, etc.) or even take a look at server manufacturer Supermicro's chassis program -> http://www.supermicro.nl/products/chassis/
I also have this question - what is form factor for the HP Proliant server boards? In particular, what is the size/form factor for the motherboard in DL380 G7 (intel 5520 chipset, dual LGA1366)?
