Go to your local feed or garden center store(s). They should stock it or you should at least be able to order it from them. You want food/feed grade DE nothing else.
Just for grins you could go into your wally world and say "where is your feed grade DE"? They will probably send you to the grocery section.
We own a Nursery and Garden Center in BFE Texas and stock several sizes.
Feed Grade DE is a natural insect killer for your garden or flower beds. Mix with your feed for your flock and dust the floor of your coop after cleaning before adding new bedding. Helps with mites and definately does a great job with the smell factor.
You can search the group for posts on DE as to amounts per, but likely you would need to adapt amounts to your specific location and coop design.
As an example here:
3 lbs feed 1/2 handful of DE mixed
Coop floor:
Liberal dusting in high humidity, less with lower humidity
Scratch treats:
Just a tad mixed in.
Forgot to check your location before I replied. but we purchase from a wholesaler that specializes in selling to Mom&Pop to larger Garden Centers. Would be happy to find a local dealer in your area if there is one.
Hope this helps...