The Mercado Modelo Turístico is at Calle Del Sol and Avenida España. Cheap amber at Calle El Sol 60, recommended.
Outside town, on the main road north and south, are many potteries where ceramics can be bought very cheaply (how you get them home is another matter).
The native amber is sold throughout the country. Do not buy amber on the street, it will as likely as not be plastic. Real amber fluoresces under ultra violet light (the shop in the Amber Museum in Puerto Plata has a UV light); it floats in saltwater; if rubbed it produces static electricity; except for the very best pieces it is not absolutely pure,...