OK I’ll try once again to give you some resources. Just Horses, google that, is an online and in the tack shop magazine listing horse professionasl, barns, farriers, etc. in New England.
Equinenow dot com is an online mag which lists boarding barns. Look under dressage for barn types, trainers (you can ask their recommendations)
Google Maine Dressage. You will get the maine dressage society, and they have several chapters. You can get a listing of shows and clinics, which should give you names of trainers, and barns hosting clinics. You can call them for recommendations of places you can board.
Those three resources should give you some clues to get started, hopefully. I know My dressage trainer, in Connecticut, drives up to (some part of maine) once a month to give lessons to three or four people in Maine, (at least she did, three years ago, I have moved away) and another chunk of people in NH, so trainers really do know barns and barn owners, including some more private places, who might not be commercial. However, I happen to know that the ladies she goes up to teach have their own private indoors, each, with like a say 4, 5, 6, or 7 stall barn, with some who have one or two boarders, so its worthwhile to ask trainers and clinicians. Call farriers and ask them. Call vets and ask them.
Hope this helps. Now to see if COTH actually will let me post.