Check to see if there is a capictor on the fan motor. If there is you can check it by removing the wires from it, using an audible ohmmeter touch both the contacts on the capicitor, your ohmmeter should go immediately to a high whistle or whine and taper off to a low pitched sound.
See if the fan turns freely with the power off. Follow all the wires to the fan and make sure they are all connected. If the unit is over 10 years old you may need a new motor, take the old one with you when you shop for a new one.
You may want to try United Refregeration, Grainger or ebay.