Cheapest way to do this:
Buy the 12v CPU fans of your choosing. The more expensive ones will run more quietly, keep that in mind. Then use an old cell-phone charger, or any AC adapter that is 12v, and splice the wire from the fan to it. Voila, you have a small fan setup that will plug into a standard outlet. Plug that into a timer and your ready to go. The other way of doing this is to use a computer power supply, but thats more money and another thing you have to find room for inside your box.
Remeber to seal any exposed electrical connections with heat sink or electrical tape. We don't want any fires or electricutions.