I order food grade diatomaceous earth from earthworkshealth.com
I order from them because they send the DE in a large plastic container 2.5 pounds, I think. And the large plastic container is durable and your DE does not burst open in shipment. I re-use the containers all the time to pre-mix my chicken feed concoction. My chicken feeder holds exactly one DE container of feed. You can buy 4 of the containers at a discount. I have been using it for 2 years - I put it in my pudding, my henhouse, my hen pen, the area my chickens dust themselves in, etc. and my own house to get rid of any mites, roaches or other bugs.
The dust is very fine, so I put it beneath the wood chips in the hen house and it keeps all the mites out of the henhouse and off the birds.