Riverhead's shopping choices have really taken off in recent years - you will find a Stop and Shop, Waldbaums, King Kullen and a BJ's along with many additional specialty food stores. A new Super Walmart and new Cosco are currently under construction.
Here is a link to the Riverhead bus schedule, along with a map of where the bus stops are. http://www.sct-bus.org/schedules/8a.pdf
All the best!