Carl has been wearing the same hat for 5 seasons of The Walking Dead now. Given that he's been at the age where much of a person is continually growing, why does this hat still fit him? Was it ever explained on the show that a new hat was found?
As much as I remember he was given the hat by his father. It was actually Rick's sheriff hat. So it already is an adult man's hat and I doubt Carl has already outgrown his father in terms of hat size.
Carl is using Rick's sheriff hat. When Rick sees him using it at first, he mentions they need to adjust it to his head, since the hat was not sized for Carl, and then takes off the sheriff badge. Now, I don't know about sheriff hats, but I've seen people using some filling around the edges to make a cowboy hat fit.
(This is speculation) I'm guessing they did something similar to his hat. As Carl grows, they just removed the filling.