I would have thought that, like with most things, the ferries and ports will see this as a money-making opportunity and charge you ten times what you would pay elsewhere for the same thing!
Some information from an English friend who has lived in France (Paris) for many years has said that:
Apparently there's a severe shortage of the kits because there are
only two manufacturers in France. They need to be NF (French version
of BSI) certified before they can be sold here, so I think that counts
out most of the international competition. I wish I had shares in the
French companies. I bet some ministers have.
At the end of the day, it's a fine of €11, FFS. About the price of two
beers in Paris, so bollox to it.
and another Francophile summed it up nicely:
It would seem the only people taking this new law seriously are Brit