You are probably going to have to get your parts from the dealer. I believe they are like $15 a piece. Replace both of them if you are going to take the time. The idler and the belt you can get from Napa. The tensioners are pricey, I want to say like $100. You will need to know whether you have a 105A or 130A alternator to get the correct belt. Since you live in FL and you are spoiled with nice weather, id just get your idlers and drive to Napa and get your other parts and do it right in the driveway......that's what I did. Take a 1/2" ratchet and a pipe to relieve tension of your belt, pull it off and swap parts. Refer to the diagram on the plastic cover under the dash to put your belt in correctly.
2011 LML 3500 Dually
5" Flo Pro