Here are two links for corset making materials at really good prices. At http://www.dragontowncorsetsupply.com/ the prices are less expensive than the other one if you are only buying a little boning.
you can buy in bulk and reduce the cost. Also they have all the materials needed for making a corset (i.e. busks, lacing, grommets etc.) I personally recommend Corsetmaking.com to buy the supplies from.