How long have you had the boa? Or better yet, how long have you had any of them? Who was the latest aquisition? The mites probably came in with that one.
Mites have a nasty habit of hiding or laying eggs under the eye rim of snakes and there they wait. Once the eggs hatch, you have a mite problem.
You are about to embark on the reason that I HATE mites and fear them. You are going to have to clean EVERY cage in that room and treat EVERY snake even if they don't show any signs of mites now. WHY? because if you don't, mites can pop up later.
My advice is to buy some Provent-a-mite. Clean all the cages, hideboxes, etc with a 10% bleach solution. Use newspaper for bedding for AT LEAST a month. Treat the cages with the Provent-a-mite. Place the snakes back into the cages and wait for the mites to die from the Provent-a-mite. (Doesn't take long) I usually retreat once after a week. Then again after 30 days. I would also treat the carpet in the room with a Carpet Flea Killer.
Everything that comes into my collection gets quarentined in a separate room and treated with Provent-a-mite. I had to deal with a nasty outbreak of mites in a pet store where I worked. I fought for MONTHS to get rid of them because the boss wouldn't breakdown and buy a can of Provent-a-mite. This experience has made me mite paranoid. The paranoia has paid off though. I have had a couple of snakes where mites popped up while they were in quarentine but because I use provent-a-mite from the beginning, the mites never got a foothold.
Good Luck