John Deere is not the company they used to be. I doubt they have a tensioner and if there is it has little, little adjustment. If the belt keeps breaking clean you better bring it in for some warranty work. If it shreds everytime your working it too ...
Call the John Deere house in your area...they will know this...most John Deere Dealers only work on John Deere., and in my area, we don't have those..Texas...but, any John Deere House will be able to tell you...I can work on the engines...anything el...
check your local classifieds; check with a dealer who takes trade-ins. Check with an on-line site such as Craigslist or the auction sites for completed sales.