I just got a buckwheat zafu that I ordered from an Amazon marketplace seller in the UK. It's very high, and the buckwheat is solid.
I have a lot of trouble sitting on a cushion - at least the one I have - because of arthritis in my knees, and the cushion not being high enough gives me back and neck pain. I could immediately feel, trying this cushion out, that not only does the additional height make me more comfortable, but the buckwheat gives a much more solid base. (I've been sleeping with a buckwheat-filled pillow for more than ten years, and it's wonderful.)
So, anyone who finds a standard zafu uncomfortable might want to try one like this...