Contacts special Branch Police. They are on the super highway near those telecommunication towers past Big c. it's the second building from the front. Upstairs on your right as you reach the stairs at the very end. Bring all your documents. The officer handling applications is very friendly and diligent. Once all your documents are in order then he'll have you fill out an application and give you an interview on the spot.
Yes people have gotten citizenship here in Chiang Mai but it's not easy as in Bangkok.
I'm going through the process now if you have any questions.