You've found it; you're quest is over. Liquors Hasegawa has one of the best selections of bourbons and Scotch in Tokyo and you can sample most of the bottles for a nominal fee.
Hasegawa is located in the Yaesu Underground near Tokyo Station. The easiest way to get their though is to ride to either Nihonbashi or Kyobashi (both on the Ginza line) and then use entrance #23 or 24 to enter Yaesu Underground. Hasegawa is down the corridor from these entrances (there is only one way to go). It is possible to navigate to Hasegawa from Tokyo station, but it is a very long walk and there are many twists and turns that are difficult to navigate on your first visit. More information can be found on their website.
Hasegawa has an approximately eight shelves of bourbon that go from the floor to the ceiling. The most expensive items are located on the top shelf and hard to see, but it is fairly easy to visually browse the rest of the stock. There is a stock list printed in katakana available, but the stock list on the website is kept up to date and includes a picture with each entry. Hasegawa has every widely available Japan-only bourbon as well as many bottles imported from the United States. The best part of the store, however, is that most of the bottles are available for tasting.
Each bottle that is available for tasting will have a special sticker listing the price for a taste (approx. .5 oz./15mL). These prices start at JPY 100 and top out at JPY 200. All in all a pretty good deal. When you do decide to taste, one of the clerks will pull out a shelf from shelving unit on the opposite side of the aisle and pour a small amount of bourbon into a stemmed spirits glass. Hasegawa limits the number of concurrent tastings to two, but there is no hard limit to the number of bourbons you may try. There are, however, signs that state that Hasegawa is not a bar, so don't get disorderly.
The only downside of Hasegawa is the store itself is small and most of the space is taken up by shelves full of bourbon, Scotch, calvados, gin, etc.. There is only one very small aisle and this can get crowded on weekends, so I recommend visits on weekdays.