Buying a new phone battery or new phone

My phones battery gave up the second day on my two week trip here in Japan, I tried to go the big store yamada denki with out any sucess, they said it wasn't possible because my phone was swedish, that the battery was diffrent, aren't all samsung galaxy s4 mini batteries the same?
Should I approch them diffrently next time? is it even possible to buy a new phone battery in japan?

Or where can I buy a shitty phone for the two weeks, I have a docomo sim data card in my phone that i bought for these two weeks ...

someone please help me...

S4 mini are not officially sold in Japan so big electric store may not carry the battery.
Technically, all S4 mini models should carry the same battery specs.
Where are you in Japan?
Your best bet will be to tryout Akihabara, where they have many shop specialise in import , simlock free phone,2nd hand phone.
http://iosys.co.jp
They sell electronics including phone and accessories.

Another alternative will be to use those USB portable battery pack. Just connect ur phone to it with USB cable. Maybe troublesome. Get at least 3000mAh for double s4 mini battery time

Or amazon.co.jp get it deliver to your hotel or nearest convenience store. You can pay by creditcard or at convenience store.
https://goo.gl/OhcZeT
S4 mini OEM , not original battery at amazon.co.jp

Big electronics stores typically don't do repair service. You should find an independent repair shop instead that is near where you are staying. Just look on the internet, they're pretty common in the bigger cities. I've actually had to do this myself just last week for a broken iphone battery and the replacement was fast (15 mins) and relatively inexpensive. I don't know specifically if they will carry the model battery that you need, but it's probably worth dropping by to ask.

Just use a powerbank to charger the phone on the move, or use your power adapter and charge it in the hotel.

That could be an option too, depending on how the galaxy handles the type of battery failure that you're experiencing. However, in some cases a battery failure can render the phone completely unusable.

The problem is the op's hasn't giving much detail about his situation, going from past experience, the battery don't just die out and don't work, it should show sign of wear and tear, or not holding a charge or fast draining.