I presume you mean the kind of black hat with a wide brim that is seen in pics of the puritans, and also generally worn throughout Europe for a couple of centuries, with variations in style created by pinning up the brim in various ways.
Here is an ECW woodcut showing everyone wearing this kind of hat.
The tricorn hat of the early 18th century was the same hat with the brim pinned up in three places.
As infinite_array indicated, it is sometimes known as a beaver hat because it was often made of felted beaver fur, but could have been made of felted wool. I have no idea what the people of the time called it.
If you want some of these for converting an
IG force, then wargame figures of the 17th century are the place to look, I reckon.