Both hakama that I own and regularly wear currently are 100% cotton (one from
Bu Jin Design and one from
Iwata). The Bu Jin hakama is a very heavy cotton and the Iwata hakama (indigo) is of a lighter material.
Cotton hakama tends to be a bit more "high maintenance" in my experience. You really have to learn how to fold the hakama correctly including all of the "inside" pleats; otherwise, the hakama tends to learn to just bunch itself into a huge ball of fabric...
I wash my hakama occasionally (once a year or so?) in the tub. Otherwise, I just let it hang out to dry in between keiko. Washing a cotton hakama in any kind of hot water will, of course, inevitably cause it to shrink. You can get the hakama dry cleaned to avoid shrinkage although it's sometimes difficult to find a dry cleaner who knows how to put the pleats back into the hakama after washing...
-- Jun