You won't be able to buy the European Apple 20 W adapter in the U.S., but you could buy the adapter once you get to France at an Apple Store or shop that sells that Apple goods. The advantage here would be that the power adapter will fit in the standard European wall plug that is used in France.
The disadvantage would be that you will only be able to use the power supply in Europe, because the supply will not fit into the standard U.S. wall plug.
An even simpler solution would to purchase an adapter that would allow you plug in the U.S. type plug on your current power supply and then plug the adapter into the wall socket in France. Then, you could use the same power adapter in the U.S and in France.
The adapter would look something like this:
Amazon has them as do the shops in the airport in France. Better hotels almost always have adapters like this as well.