If you use a computer fan all yoy need is a 12v power supply. The amps or milliamp does not matter as long as its 12v. You then splice the black and red wire into the power adapter wires. If there is a white and yellow wire on the computer fan you can leave them disconnected for high or splice the yellow and white wire together for low/medium fan speed.
You can also get 110v or 220v fans the size of computer fans but usually they are more expensive and people usually have a computer fan and 12v adapter laying around doing nothing.