Boy, I have been on every street in the town and if there is anything like a civil office I haven't seen it. Good question. The only official buildings I can recall are the PN and Politur stations (PN has the courts as well) and the school district office adjacent to the big school. Are you sure they have one - you can try the superpages for the DR and use the term (hope this is spelled ok) - wait here it is:
AYUNTAMIENTO MUNICIPAL
Direcci?n: DELEGACION BOCA CHICA BC
Tel?fono: 523-4221
FYI the link is
http://www.amarillas.com.do/gobierno/gobierno.asp
Pick Region este and don't enter anything to search and you get all the govenment listings.