Your local home center or big box store will fit that bill nicely. Fans can be had for as little as $50. Just remember if the light fixture did not have a fan on it already, the support box that normally can hold a light fixture may not be nearly as strong enough to hold up the weight of the fan and the movement that vibrates to it when the fan is running. Check with the management or owner before you spend any money on this to be sure it can support the new fan.
Have you tried your local Habitat Re-store? Great finds for little $.