Hi,
I'd do a bit 'o' research like the guys suggested, but as you pointed out, availability can be a problem. Rapidonline.com has plenty of components, cheap, as does farnell.com and rswww.com.......the last two have minimum orders though. It's all very well finding a 'perfect' match, but if you cant get hold of the bugger then its not much use. At a guess I'd say any N-channel MOSFET, with *similar* specs would do...it is a graphics card after all, not a high powered inverter, check if it has inbuilt diodes though, some do, some don't...I'm not really up on MOSFETs.
Gah, I need a decent graphics card...wonder where I could get hold of some decent comp parts cheap...repairing them would be a challenge (yes, I'm sad).
Good luck,
Blueteeth.