As the prices of currencies oscillate all the time with respect to each other, there is no best moment to order travel money (in this case yens).
It is advisable to pay attention to how the economy of the country whose currency you are interested in is evolving. In general, when the country´s economy is tough, its currency tends to depreciate with respect to the rest. This means that for the same euros you will obtain more yens.
Comparer Devise displays the sell rate of yens, arranged from cheapest to least, to help you decide where to buy yens.