There exists, where you would least expect it, a marvelous oasis for sake shopping. Yoshiike department store, just outside Okachimachi Station and just across from the entrance to Ameyokocho, has a fantastic selection of sake and a plethora of sake utensils to go with it.
The store is directly outside the Yoshiike Exit of the JR Yamanote Line station, and just around the corner from the Toei Oedo Line subway station. Yoshiike's been around a long time and has a very retro feel, but despite being as glitzy and polished as most department stores, a little corner of sake heaven awaits you at the top of its first-floor escalator.
The entire second floor is dedicated to alcoholic beverages and their accouterments, with the lion's share of that being sake. The nihonshu in stock, easily several hundred different sake products, fills a wall and a half.