craftgrrl | alternative to styrofoam balls

i 'm using styrofoam for heads on dolls, it works great because it weighs almost nothing and i can stretch the knit fabric over the ball and get a perfect shape. however, i don't like using styrofoam knowing it is bad for the environment and it isn't cheap either. i tried a yarn ball but it is too heavy. does anyone know of another product i could use instead or have any other ideas for it?

i've thought of a papermache ball or a rubber ball but both have problems. the papermache would be very time consuming to make and i'm not sure i could get a perfectly round shape. a rubber ball might work but i do a lot of sewing with these dolls so there is a chance i could poke a hole in the ball. i'm really hoping there is some magic good for the earth alternative to styrofoam i just don't know about.