I ventured out to Active Surplus here in Toronto, Canada. It's on Queen West, if you need to know. They look like little black frames-more or less like the fan on the back of your machine. Good brands are Boxer and Panaflo. First, make sure it's a DC Brushless type.
Brushless fans generate less electric noise, meaning better for computers.
Next, examine what kind of voltage and what kind of power draw it has. 0. 10 Amp model. Your Amiga should be able to generate both 12V and 5V. DC, by the way. Look at how much air it blows out. Air flow is measured in cubic feet per minute.