...you could do a lot worse than to unscrew it and take it down to Maplin (if you have one near you) and they'll do you one there.
That's what I did and my PC sounds sweet now.
Which fan is it? (Mine had about 4!)
Alternatively you could search net for it.
Once I'd found out the voltage, connector-type (2, 3 or 4 pin etc.) and the size (40mmx40mmx10mm for me) searching for one becomes easy but I went Maplin in the end - 4.99 and it was sorted.