As mentioned by others, look for sites that carry supplies for people working with textiles. You will find that it goes by various names - mine is call Pro Thiox. I got mine from Pro Chemical and Dye http://www.prochemical.com/catalog/auxiliaries.htm
I see it is going for about $11.00 US per pound at this time. They sell it in half-pound size also; unless you are planning to do a heck of a lot of toning, this would probably do you. A little goes a long way.