I just went out to Bean Farm and picked up a few cans as i live close to them. You can order it from Reptile Basics, Bean Farm or any other reptile place. What you might want to do also is since you cant move the snakes, just spray PAM on a rag and wipe down the outside of the cages of your other snakes as well as any cords or cables and table legs etc. Anywhere mites can crawl up to and reach the snakes. Then again, wipe down the outside of the cages especially around vents and doorways. This will give you a double layer of protection. I would also just give the the other snakes a one time treatment. You can spray it directly onto substrate but i would avoid plastic plants and stuff where it wont be absorbed and dried fairly quickly. Also i would leave the water bowls out overnight after treating the cages. The snakes wont die without water but if they find a spot that wasnt completely dried and crawl through it and then into their water bowl and drink, you will have problems. I had to take my JCP in on an emergency to my vet when that happened. Luckily the vet flushed her system with saline and saved her. But just leave the water bowls out overnight.
And again, i wouldnt worry about the humidity. You want to get rid of the mites first and foremost. Having 50% or so humidity for a couple months wont hurt the snake as long as it has access to water.