If you are at the Jigokudani Snow Monkey Park, Kanbayashi area, or Shibu Onsen area, there is no ATM around that accepts international bank cards. You'll have to travel back to Yudanaka to get access to one of those. But as soon as you get to Yudanaka Station, you can find an ATM at one minute from there. There are two places where you can get cash out of an ATM with an international card: the post office and the Seven Eleven. The Seven Eleven is about 20 minutes away from the Station by foot, but the post office is easily accessible. See the below map for the location of the post office.