Foreign Currency Exchange

1. Exchange some cash before arriving
2. Avoid exchanging currency at airports or near tourist sites
3. Use ATM machines to get the best exchange rate available
4. Use credit cards for bigger purchases

Where to get foreign currency in Japan

Many automatic teller machines (ATMs) in Japan do not accept credit, debit and ATM cards, which are issued outside of Japan. Among the ATMs that accept Maestro cards with IC chips are 
1. Post office (over 20,000 nationwide or at family mart stores nationwide) 
2. 7-Bank ATMs (found at over 10,000 7-Eleven stores nationwide)  
3. Aeon Bank ATMs (found at Aeon malls )

 These ATMs allow you to withdraw cash by credit and debit cards issued outside of Japan, including Visa, Plus, Mastercard, Maestro, Cirrus, American Express and JCB cards and provide an English user menu.

also, 
Intrnational airport
Major department store (Takashimaya, Mitukoshi,Isetan,GinzaSix)

If you are visiting outside of major cities, please note that there are much less convenience stores and Postal ATM operating hours are different. 
It is basically depending on thesize of the post office. Typical postal ATM closed at 18:00.