I'm sure I have one (or 6 or 10) laying around. Aren't there particular voltages/wattages etc for what will work? I remember frying a board with one that was too potent. Oops.
You just wanna match up the phone charger with the fan. Like for example a 12 volt fan goes best with a 12 volt phone charger. You could go with lower voltage on the charger but it would make the fan spin slower.