You don't need pure sulfur to cast objects. I would use sulfur used to produce the caps on concrete cylinders used to test compressive strength. They sell the bagged version of this stuff (http://www.durhamgeo.com/pdf/msds/Ca...d%20Cyclap.pdf
) at Durhamgeo. SoilTest has their own brand. It will work fine for casting with the filler in it. It comes in various colors but not bright yellow. I assume that the color is a secondary concern?