As the rates of currencies vary all the time with respect to each other, there is no best moment to order foreign currency (in this case dollars).
It is recommended to understand how the economy of the country whose currency you want to buy is evolving. In general, when the economy of that country is hard, its local currency tends to depreciate with respect to the rest. This means that for the same euros you will obtain more dollars.
Comparer Devise displays the sell price of dollars, sorted from cheapest to least, to help you choose where to order dollars.