Sewing machines are plentiful, even in Sosua. I just saw a Singer basic machine in the shop attached to EPS across from Britannia. Top shelf, way in the back. They have all kinds of small appliances there. Things like electric razors, automatic drip coffee makers, electric griddles, irons, blenders, cd players and mucho mas!
In Puerto Plata, on John F Kennedy, a shop called "Mi Regalo" even has sergers. Also, Gonzalez and many small shops all over Puerto Plata. Just ask around.