There is a Metro Bus stop/ticket counter beside the farmacia near the Texaco at the Corner of the autopista and Calle Dr. Rosen in Sosua.
IIRC there are only about 4 departure times daily:
8:20 am
10:20 am
1:20 pm
5:50 pm
Since it at least a 3.5 hour ride (more like 4 hours) to the main Metro terminal in Santo Domingo and another 30-45 minutes taxi ride to the airport I'd count on 5 hours total time one way. If you want to arrive a minimum of 2 hours before your flight you'd have to leave 7 or more hours early from Sosua making the 10:20 am bus the last possible option. It might not hurt to consider the earlier bus incase there is a shortage of seats although you can prepay for the later bus at the office in Sosua.
I believe the bus is currently about 300 pesos oneway to SD and a taxi from the SD bus treminal to the airport should be between $30 and 40 USD. Total time 5+ hours and total cost $40 to 50 USD.
Metro Servicios Turisticos