If you need help in knowing where to buy a specific item in Japan, check out the Deep Japan website which has tips on all sorts things related to shopping, from the ever knowledgeable senpai!
From popular Japanese souvenir or gift ideas to advice on the international supermarkets in Tokyo, you are sure to come across useful information about shopping in Japan here.
An example of one such useful article on Deep Japan is an piece written by Joe Peters that talks about the different international supermarkets in Tokyo: http://www.deepjapan.org/a/1476