Small Engines (Lawn Mowers, etc.): finding the right belt, model numbers, automotive parts

Question
I just bought a john deere sabre 15.5 horse, 38 inch deck mower. The belt just broke and I am trying to find a replacement. The belt that was on it I thnk was wrong. Where do I go for a replacement and what size do I ask for?  

Answer
 Hello Jared:

 Send me the Model Numbers Off of the Mower, Please. With these I can Locate the Belt Numbers you Require and then you can go to the Local Automotive Parts Store and Purchase an Industral Belt that will be the Same or Better than the OEM Belt. Thanks.

 Respectfully

 John