Currency exchange to Japanese YEN
You can exchange to yens before leaving, in your country, or directly in Japan.
The cheapest solution is often to withdraw cash in Japan through a credit or cash card, but be careful because:
Not all cash and credit cards work in Japan, you might have some problems, for further information read this page
There may be fees or rates on withdraw abroad, and of course they depends on the type of card and bank you have.
Ask your bank about these aspects and realize whether it is really convenient or not.
Having a bit of cash without relying completely on withdrawals, however, is recommend to anyone.
I can't talk about currency exchange in your country because it is a worldwide website and I live in Italy, so let's see where to exchange in Japan.
Exchanging in Japan, unlike what you could suppose, is quite convenient.
It is very difficult to find scandalous exchange rates, as can happen for example in my country (Italy) or happened to me in Us (New York), even at Narita or Kansai Airport you can find very good rates.
For example at this link
you can find a map with the location of the various offices of that forex company scattered around Tokyo and other cities in Japan, which have very affordable exchange rates.
At this other link
the current rate they charge.
The first "CASH" column indicates the rate of yen purchasing (ie if you have some yen and want to exchange them), while the second "CASH" column indicates the rate of yen selling (ie what interests you when you arrive in Japan and want to exchange your money to yen).
The "T/C" columns, where there are even more affordable rates, indicates the exchange rates of traveler's cheques.
The Traveler's cheques should be also a valid alternative, but I've never used them and I can not tell you more, inquire with your bank.
Even better rates can be found in the Japan Post offices
(open Monday to Friday until 16:00), that can be found also inside airports.
In Narita airport, terminal 1, the post office is on the 2nd floor, just one floor up from the arrivals floor. During my last travel I exchanged right there, there was nobody and I did immediately, the official stock exchange rate was about 1 euro = 103 yen, while they applied me an honest exchange rate of 1 euro = 100.15 yen.
Alternatively, you might find good exchange rates also in banks
If you really need to exchange money, don't be afraid to use even currency shops, as I said earlier it is rare to find very unconvenient exchange rates. It is the most honest country I've ever visited as regards the exchange of foreign currency.