It’s a style Lee wears in a few westerns. Also known as the Montana peak. Check out Monty Walsh if you haven’t yet. Looks like there are some cheap options out there. Stetson makes one in fir felt but it’s almost the price of a custom made rabbit felt hat. I would personally keep an eye out for a cheap old western fur felt hat and modify it yourself.
Johnny
Hell, yeah! I love those campaign hats too. They were standard issue in the US Army from the Philippine Insurrection right up until WW2. And, of course, the "Round Brown" is still the designated headgear of US Army Drill Sergeants. My guess is that those are made by Stetson, as Brother @alaskamcghee noted. Plenty of state troopers and park rangers still wear those hats too. So they're out there.
As I understand the history, in the 1800's Stetson and other hat companies only sold hats with undented crowns. Then the cowboys and ranchers in the West creased and dented the crowns based on whatever was popular in their locale. You could tell who was from which ranch or town based on how their hats looked. Naturally, the four-dent style became popular in Montana, hence "Montana peak".
As far as actually wearing one casually, I think if you wore a crisp hat with a stiff brim it would definitely come off as costume-y and you might have to endure jokes about being Smokey Bear or Dudley Do-Right or somesuch. But with a DIY peak, ala the original Montana peak cowboy hats (hat tip to Brother @alaskamcghee again) and/or a stylishly sloped brim, it could work as a quirky alternative to the more common modern Western hat styles.