As best as I know, there are 2 busses that leave Sosua at 6 Am and 7 Am respectively, from in front of the Sosua polica staion.
They are big busses, but not as luxurious as the Caribe tours bus.
They wll drop you off in Sanchez in 3 hours plus, then 30 minutes of public transport will get you the rest of the way.
Will you be traveling from Puerto Plata or Sosua? The buses that Rocky referred to start in Puerto Plata and pick up people waiting along the road as they pass by Sosua on the highway. The one I take to Samana passes by about 7 am. You can take it as far as Sanchez, the town before Samana, then get a guagua from there to La Terrenas.