Where is the best place to purchase KitKat candy bars in a variety of flavors?
In the United States, we only have milk chocolate and maybe matcha green tea if there's a local Japanese supermarket. I want to try flavors only found in Japan!
Several unusual tastes can be found in Don Quixote ドン・キホーテ (Roppongi, Asakusa, Akasaka), if you are in Japan at the moment. Also now there is an Easter KitKat at many supermarkets (apple and carrot taste). We visited the official KitKat store but there is not much choice. BTW, in Kyoto they have hojicha KitKat and Uji matcha KitKat.
Every time a new seasonal KitKat comes out here you can find that and others at supermarkets/ convenience stores throughout the country. I read in Metropolis magazine that there is a carrot and apple pie variety coming out next :) Also, I've noticed at Narita airport in the terminal they have a wide selection I guess for people taking home gifts.
Tokyo JR Station also has a lot of souvenir shops within the boundaries of the gantries itself as well as in the basement shopping street carrying scores of regional snacks, confections and souvenirs. These same shops also carry the seasonal range of KitKat that would be available.
I'd recommend visiting Don Quijote. They have so far the cheapest chocolates, and you get tax free (when you present your passport) purchase as well. However, it depends on the branch, some shops have limited flavors of Kit-kit, while those bigger branches (mostly in Tokyo area) have several flavors of Kit-Kat. It's worth a visit.