Dongwon Beverages in your local supermarket. Except for the blck tea, which is slightly sweetened and tinged with lemon, they're sugar and calorie-free, made with only pure tea and mineral water. I've heard some say that they are brewed a little strong, but in my opinion, they're perfect. You can get black, green, jasmine and chrysanthemum teas.
They're North-American priced at 1600 won, but a whole lot healthier than a Pepsi.