wiwu

The auger belt keeps slipping off from the auger pulley (the bigger one on the lower part) starting yesterday. No visible tear  and wear on the belt. Adjusting the idle pulley did not help -- after a few minitue running, the belt jumped off when the clutch was released. I had this unit for a couple of years. Is it time to replace the belt? I called Sear Parts/Service center and they told me they do not stock the belts. Where can I buy the belt?  How to replace it (I lost my manual)?