You can buy 99% pure food grade sulfur on Amazon. BUT! I just used 90% pure garden dust ;-) which cost me about $7 for a two pound shaker can at a local nursery. This brand was ferti-lome Dusting Sulfur. It is sold for all sorts of fungal diseases on garden plants and for the control of thrips and different kinds of mites on plants. It is not listed for use on animals, but I didn't want to wait for the food grade stuff to be mailed and took a chance. Label says 90% sulfur and 10% "other ingredient". I don't know what the "other" stuff is, but figured if it was a synthetic poison it would have to be on the label. I am thinking that it is just the impurities in the sulfur, like iron and what not, that wouldn't be present in the more pure food grade.
Just as with using DE and wood ash, you don't want to breath it and try to keep the chicken from breathing too much of it. Because it is sold as a pesticide it has all the standard warnings and precautions on the label about ingesting it and breathing it. None of my birds had any adverse effects from it and so far I have not seen any live mites on any of the ones I have checked. Work it down through the feathers and into the skin as much as you can.
I also used it to make up some home made NuStock (
http://www.nustock.com/) according to recipe found at (
http://naturalchickenkeeping.blogspot.com/p/blog-page.html). Home made or store bought I put some of it around their vents to kill the mites, also on any sores, bare skin, bumblefoot, and if there are nits on the feathershafts I coat as much of them as I can with it. That smothers them so they don't hatch.
So far this is working like a dream. I did also shake a little sulfur into their dusting holes and into the nest boxes.