Find out the voltage rating of the fans you plan to purchace then if it's something easy like 9v, get a 9v AC-DC adapter (Assuming the fans are DC) and strip the wires, and the fan wires, then get 2 wire nuts, and connect positive to positive and negative to negative, then connect them to the hottest metal part of the amp.
If it's something else you may be able to find a portable adjustable DC power supply with aligator clips, and do the same.
If you don't want to damage the fan wiring, you can get a small computer power supply for about $20.