I gave up finding food grade locally and settled for plain old diatomite from a pool store a few towns over. I have been feeding it for 6 months and have had no mites, no worms, and no deaths. That stuff weighs so much it was unpractical to get it shipped in as much as I would be using.
I spread it on their litter, in their nesting boxes, and mix a few scoops into their food bin too.
Finding it around here was imposible so I was happy to find it from a place I trust online. I ordered a few pounds of it. Some for the dusting area and some for the feed plus some to put back for later.
My husband loves your avatar. He's a huge fan of ATHF and the Venture Brothers. He was looking for mooninites (forgive the spelling) pics the other day.