I bought my replacement belt at Autozone. I just gave them the make, year, model, and engine (Ford, 2002, E450, 6.8L) and they looked up the belt. You can look it up on their web site with the same information. They had two different belts: Dayco, and Durolast. The Durolast is their own brand while Dayco is available at other places. Other people might recommend Goodyear or OEM, but I have used Dayco belts before and never had a problem so I went with that brand. As mentioned earlier by Dusty, if you are trying to fix a chirp or squeal, replacing the belt isn't a guaranteed fix.
r/ Mike