The source I read said, "a handful," so I put a heaping cupful (measuring cup) in. I think I'll add more.
I checked a garden center for sulfur (it has many uses in the garden), but what they had was granular, and I wanted the powder.
This is where I ordered it:
https://seedbarn.com/products/sulfur-powder-10-lbs?variant=12544546209892
$20, plus USPS postage came to about $29.
Just FYI: sulfur powder is a very fine powder. I have it in a couple of plastic containers with snap lids and put a note on them to "open carefully." I don't want to spill and have that stuff poofing out everywhere.
I also put some in a sock and stretched that across the pop door opening of the coop. It rubs the chickens' back as they walk under it. I knew that they would be afraid of "that scary thing in the doorway," so the first few mornings, I only opened the pop door and not the people door so they would have to come out that way. Once the first hen did -- and nothing happened! -- the rest eventually followed.