I'm not familiar with your particular model, but Dell normally builds machines from standard components. I suspect you would be able to get a replacement fan from somewhere like Maplin Electronics, however, you could try this first.
WARNING. Opening a PSU exposes you to potentially LETHAL voltages and should ONLY be done if you know exactly what you are doing and the machine is TOTALLY DISCONNECTED from the mains supply. Capacitors within the power supply can hold voltages for a considerable time after disconnection.
Often, a noisy fan can be given a new lease of life just by lubricating the fan bearing. To do this, gently peel back the foil cover on the end shaft of the fan, and apply ONE drop of light machine oil (3 in 1 or sewing machine oil is ideal). Once reassembled in the computer, let it run for a small while and the noise may stop. Alternatively, a replacement fan may be available a indicated above, but often, the localised connection may have to be re-made to connect it to the PSU assembly. Obviously, correct orientation of the wires must be observed.