Japan is innundated with convenience stores like AM / PM, Family Mart, and even 7-Eleven. Indeed they are convenient, but these outlets have a restricted variety of goods. I found my favorite grocery store to be Akafudado, not far from the great Ryokan Sawanoya. There are three levels of mostly Japanese products on sale, with just a few Western products (mainly chocolate bars and snacks) in case you get homesick. This is a great place to look around for new and unusual products if you have curious taste buds.
It is a well-stocked grocery store, so this is a perfect place to load up for your typical self-catering food and drink needs. There are baked goods, fresh produce, dairy products, liquid refreshments, and all sorts of interesting packaged goods. How about some seaweed crackers? Energy drinks with weird names like Pocari Sweat? Chips that are shaped like pea pods?
Akafudado is very close to the Nezu Station on the Chiyoda subway line.