For browsing, you have to visit the Ginza flagship store. It has a wall of chain belts!!!! I love chain belts so the first time I saw it, I was gobsmacked! Plus there are always interesting exhibits on the top floor. If you are looking for bags from one or two or even three seasons ago, as already mentioned by others, the concession at Isetan in Shinjuku is the place to go. The price however is at least 30% higher than US or HK so that place (in fact any Chanel boutiques/concessions in Japan) is really your "last resort" place!
As for Hong Kong, the more popular Chanel gets, the more difficult it is for plain Janes like me to get the bags I want. And since the price between US and HK appear to be comparable, if you see anything you fancy in HK, just buy it!
Generally, the bags which are hot in Hong Kong may not necessarily be that hot in US and vice versa.
Good luck!