Hello there and welcome to BYC!
Ivermectin Pour On works well on mites. Depending on the size of your birds, a certain amount of drops on the back of the neck and it kills any mites on the birds. The Equine Paste will also work. A pea sized dolup on a tiny piece of bread one time for each chicken.
But you will also need to strip down your coop, remove all bedding from the floors and nest boxes, and use a poultry dust or spray and get every crack and corner. Especially the roost bar and underneath it where the bugs like to hang out until night time and come out and feed on the birds. Not all mites are on the birds all the time. Get the nest boxes well too.
You will need to repeat this coop cleaning once a week for a few weeks. You can repeat the Ivermectin as well in 10 days to get any hatched eggs.
You can also dust the birds with Sevin dust, although this stuff is debatable as to whether it is safe for the birds. Sevin dust kills everything.
Also, many time when they have external mites, they have internal worms as well. So you might worm them as well.
Good luck and welcome to our flock!