Hotels are pretty fair for exchange rates AND there are more ATM Options
Unlike in many countries, I feel that the hotel exchange rates in Tokyo are pretty fair and accurate. The best is to change money at the airport but if you need to change more later- then most of the large hotels can take care of your request (depending on currency) Also if you are using an international bank card more options are becoming available. The JP Postal banking can usually give you cash at the atm's from an international card- and also 7/11 convienience stores are a good place to use your international debit credit cards. Cases will vary wildly but it is worth trying if you are stuck.