That's the grooved idler pulley. RockAuto has them, so your FLAPS should have one too. From what I can gather, A/C-equipped trucks got a 90mm pulley, and non-A/C trucks had a 127mm.
Option - use this opportunity to switch to a SuperDuty-style two-pulley tensioner. This tensioner takes the place of both your missing pulley and the old-style tensioner. But it requires a permanent mod to the bracket (IIRC, that boss you have circled has to be sawn off). There's a Ford TSB on the procedure, floating around online somewhere. This would esp. be worth considering if your tensioner is worn as well.