There has only been one Mac specific version of the Radeon HD 7950 and it was made by Sapphire. It can be recognised by having a white plastic cover on the card, normal PC versions typically have a red plastic cover.
It is possible to 'flash' a PC version with Mac EFI compatible firmware but if you want the 'official' Mac version look for one with a white plastic cover like this one. http://www.sapphiretech.com/productdetial.asp?pid=4602E84F-4A09-4B9A-B9F9-CBA9CF 197B15&lang=eng
It is of course now discontinued but I bought one via eBay previously myself.
As mentioned you can flash PC equivalent models, this is done using a Windows utility typically by running Windows in Boot Camp, you cannot do it when running Windows in a virtual machine. Ideally you want to find one that looks as similar to the Mac one in appearance as possible, particularly with regards to the video connectors. The Mac one has two Mini Displayport, one DVI, and one HDMI connector. As you will see from above the Mac one also has a single fan.