I'd recommend getting the part number of the fan. Places like http://www.dorothybradbury.co.uk/
stock fans in just about every size and type you could want, and they'll generally have some quality ones to pick from (Panaflos and Yate Loons are pretty quiet, if they carry them in the right size.)
As for powering it, most fans of that type run on a 12v feed. Its pretty likely that the fan will say this directly on it, so check that, and while you're at it, check the fan power cable just to be sure theres no resistor in it thats reducing voltage fed from another source. If there is, make sure to, by hook or by crook, keep a resistor of the same value in the same place, even if you change the cable.
All in all not a complex job in most instances, but you're going to need to get into the amp at some point, unless you're willing to send it to a tech and get charged out the ass for it!
Oh, and when you open it, LEAVE IT UNPLUGGED (not turned off, UNPLUGGED) FOR A FEW DAYS BEFOREHAND. Power amplifiers carry and store HUGE amounts of power inside, and you can easily hurt yourself if its not left to discharge.