A Roho cushion is a cushion with a bunch of air cells in it. Sort of like little fingers. You blow it up with a little pump so that your butt is just above the hard surface of the chair and the air moves around in the chambers to take pressure off spots. Try www.roho.com They have all the info. They aren't cheap but are probably the best. Next would be the gel seat. You can google them.
AL.