I Want To Buy Diatomaceous Earth?

to deworm my porch cats. Someone on YA suggested it, but I can't find it in a small enough amount. I tried at a local farm supply but they sell it in only very large quantities and they won't open a bag and sell in smaller quantities. I want probably about 1 lb. of it. Does anyone know where this small an amount can be purchased in Ontario, Canada? I have already tried Home Hardware, Canadian Tire, WalMart, and a farm supply store.