Are you talking about a different rubber or the covering material? I would think neoprene would be the best bet for a foam rubber but I guess it depends on the density you pick.
I believe the typical material is a foam over molded with a "super soft" propylene and/or pvc type material. One way of doing it might be to have it made as a removal over-lay. Have the foam molded to or glued to the covering material and have that made so that it will snap around or to the current arm rest.
