Find an AutoZone store, or maybe order it online, but get the Dayco. AutoZone sells Dayco and it's the OEM mfg for the tensioner. It was $60. 00 for mine about 2 months ago. Don't try to get a tensioner made by Gates, it will not line up correctly and will make the belt ride off the back of the pulley, I've had the same problem with Gates tensioners on 3 different trucks.
Anyway, the "parts house" Dayco one is the same thing you'll get from Dodge or Cummins except at a lower cost.