A&D can be found at any drugstore. It is marketed for humans and is just vitamins A and D in an ointment. It is good stuff.
Unlikely. They will wither up and slough off. Don't worry too much, though. The bird will be fine after that happens. Roosting may become more of a challenge, but the bird should be fine despite that.
ETA- Mites are easy to deal with, but it takes dedication and a lot of extra work to resolve them. Dust all the birds with Sevin. Clean out the entire coop, removing all bedding and discarding it far away from your coop. Spray the coop out with an insecticide. I like Adam's Flea and Tick spray. Put some Sevin in with the new bedding, but only put a light layer of bedding down because you are going to need to do everything again in 10 days. In 10 days treat the birds and the coop again. If you have a really bad infestation you may need to do the whole procedure again in 10 more days.
Good luck.