Fans

Box or Axial Flow Fans

 * Brushless Fans
 * The common computer fan is a brushless fan. These fans are driven by brushless motors, which rather than using mechanical switching to change which coils are energized and thereby generate the rotational motion, use integrated circuits and transistors. This allows for more precise speed control, but depending on the rules you are building under, such electronics may be illegal.
 * Brushed Fans
 * Many axial flow fans are not brushless and are therefore legal every year that allows motors. The hairdryer fan featured in the description is an example of a brushed axial flow fan.

Appliances
Fans in large appliances like vacuum cleaners are AC motors are therefore not useful. Smaller appliances are more likely to be using DC motors to power their fans, though in some cases you may have to remove a rectifier circuit from the motor terminals. Hair dryers are usually a good bet, but some use AC motors. Dust busters and other similar rechargeable devices will be using DC fans.

Purchasing
All electronics surplus retailers will carry fans of various types. Radioshack and other similar stores will have some fans, but watch out for brushless fans if your rules prohibit such electronics.