Matsumoto-Kiyoshi is one of the most famous drugstores in Japan.
There are 2 Matsumoto-Kiyoshi drugstores near Shibuya station Hachiko Exit which you can find easily.
See this map. Left one is Matsumoto-Kiyoshi Shibuya Part 2. Right one is Part 1.
Part 2 opens from 10am to 10pm, however, part 1 opens 24hours.