You can get chalk boards/marker boards at Liberer?a Abraxas. Student desks and chairs were not available in Puerto Plata this spring when I was searching for them.
Abraxas is a bit expensive. There is a less expensive stationary store across from Luperon Park at the corner of Juan Bosch. Also Ripoli which is on 12 de Julio is a good source. One alternative is to have desks and chalk boards built. There is a Canadian carpenter who is in Costambar who is quite good at building inexpensive items for school causes. I know the individual has been working on a school for Haitian -- as well as doing some volunteer work at the English resource center The Meeting Place in Puerto Plata. If you contact them at info@MeetingPlacePuertoPlata.com they can probably provide you with contact info