In between Berringer Winery and Robert Mondavi Winery on St Helena Highway is a wonderful little place called the St. Helena Olive Oil Company. Here you can taste different kinds of Olive Oils that are made locally. Their packaging is plain and simple and the oils are wonderful. 
In additionn to the oils, they had wonderful dipping spices, soaps, and local crafts. Take a look at their web site at www.sholiveoil.com