Use credit cards as much as possible for the best exchange rates. When you need cash, its best to use an ATM, again for favorable rates. Look at your ATM card for a Plus or Cirrus symbol, and then go to the machines that display those symbols. FWIW, if you arrive in NRT in Terminal 1, turn right after exiting customs, and you will find a bank of ATMs at the very end of the arrival hall (just before/underneath the escalators)