As stated on the RCI website: "...North American standard, 110 volts AC is used on all of our ships. Outlet adapter is required for European standard. European standard (220 volts AC, round prongs). Stateroom outlets can accommodate appliances up to 1500 watts..."
A travel store or place that specializes in luggage ought to have adapters. I wouldn't be surprised to see them offered for sale on the ship though, as it is no doubt a popular item for European cruises and would likely have lots of passengers needing them.
I will say that on many ships I have seen one 220V outlet in the cabins which is the voltage used for the vacuum cleaners.