None of the bigger bus companies, if by that you mean Caribe Tours and Metro, have routes in the East. To go east by bus you need to pick up your guagua at the station alongside Parque Enriquillo, and if I remember correctly you travel the route to La Romana if you are headed to Bayahibe. From there you grab a cab or whatever to carry you to your specific destination. Cheapest is the guagua of course. Renting a car would be more expensive. THeguagua will charge very very little and the trip is easy enough.
