I get it at TSC! It's garden sulfur, but there are even instructions on the bag for using it on Poultry and other livestock. To incorporate it in my dust bath, I mix about a cup of sulfur into the peat moss/sand dustbath area (my dustbath is 7'x3', so the sulfur to dirt ratio is pretty low). Whenever I add more peat moss to the dusbath (maybe every 4-6 weeks), I add a scoop of sulfur.
People also put the sulfur in a sock or cheesecloth-type "bag" and hang it from the pop door sonthat chickens are "dusting" themselves with it when they walk in/out of the coop!
This is the article that I found when I first got chickens and started researching prevention (bcs of the horror stories I had read here on BYC, lol!!

):
https://entomologytoday.org/2016/07/18/battling-chicken-mites-with-bags-of-brimstone/#:~:text=The chicken sits in the,not use the dust bath.
"The chicken sits in the dust and fluffs it into its feathers to remove parasites — this is a natural chicken behavior.
Sulfur dust baths are very effective at controlling northern fowl mites, even for chickens in the same coop that do not use the dust bath."
View attachment 3890565