Any large grocery stores like la Siernna have a complete stock of most anything you desire.
As far as bus service, the Metro is the best bet in town. They connect to Santiago, Santo Domingo, Puerto Plata, Sasua, and more. The current price from PP to Sti is about 5.75 U.S. one way. The bus is very clean, modern with a LOT of air condioning. Wear a light jacket or you will freeze. Have a nice trip.