There are three sizeable supermarkets in Puerta Plata - Jose Luis, Tropicale and Messon.
I would also recommend going into town once a week for a big shop and picking up the odd thing from the smaller stores in Costambar.
Will you have a car whilst you are there? I am not sure prices of taxis into town however I am sure someone on this board can advise as to costs.
Enjoy your trip
Daydream.