2003 NOTE: I NO LONGER RECOMMEND WORKING WITH CELASTIC IN ANY CIRCUMSTANCES, SINCE NOW THERE ARE MORE THAN ENOUGH NON-TOXIC SUBSTITUTES FOR THIS PRODUCT AVAILABLE. CELASTIC IS A NIFTY MATERIAL, BUT SEEMS MORE TROUBLE THAN IT IS WORTH FOR THE DANGERS INVOLVED, THIS METHOD OF MOLDING, HOWEVER, WORKS ON MOST OF THESE SUBSTITUTE MATERIALS AS WELL.
Quite likely, if the material you're looking for is "hazardous" it probably has been replaced by something else and demand from those who prefer to "live dangerously" may be insufficient to keep in widely available. The linked site didn't give much info, but might be a starting point.