Start by looking for specifically avian vets. If there’s none, try looking for exotic vets and then call and ask if they’ll see chickens. If there’s still no help there, you might luck out with a large animal/equine vet who owns chickens and can help you out. Make sure you call and specifically ask, I knew an avian vet who refused to see chickens (no idea why). Also ask for recommendations and don’t lose heart if you get a loser receptionist on the phone who is super unhelpful.
The chances of finding one that specializes in chickens is little to none; your best bet is to find one willing to work with you and learn with you who can write prescriptions if need be.