Fedoras
They are called Fedoras.
I know they wore fedoras but I'm not so sure that they were the most popular hat they wore.
He usually wears like jean jackets, fedoras, v-necks, ect.
Johnny Depp wears many different types of hats, including Fedoras.
From what I can tell, Kendall has a big love for beanies and all hats in general; baseball, fedoras, statement. That boy sure does love his hats!
He gets them special ordered from a friend of his.
Fedoras or Trilby Hats
To swag out all of your fedoras. M'lady
Places like Hot Topic and Pac Sun. Other vintage rock shops have fedoras like him. Billabong makes fedoras similar to the ones he wears sometimes... If you type in "fedora" on Google your bound to find something that looks like his. He's work grey ones, pinstripe, plain black.
A lot of people refer to it as a Fedora - although it is much more like a Trilby (shorter brim than a fedora). A lot of people refer to Trilbies as Fedoras anyway and that's what you may see it labelled as in stores. In the live vidoe for I'm yours it looks a lot like this hat - http://www.hut-shopping.de/Harvester_Straw_Hat.HTML
Depends on what part of America and what occasion. In general, guys tend to wear baseball and trucker caps, and beanies. Girls tend to wear knit berets and beanies too. Fedoras are trendy for guys and girls, but they're rare. There are some areas of America where they wear cowboy hats, but that's more in the west and south.