If the outer casing is the problem, you can fit new ones, stainless (aka Plastic). I used a standard plastic drainpipe of a suitable diameter - you'll prbly find the same size in France (eu standard - iso etc.). This can be done without removing the shocks, just take off the bottom mount, and remove springs (or whatever it was i removed ). Cut off remaining casing - leaving only the top sylindrical (straight) part. Cut a split (or 2) in the plastic pipe and slide it on, together with a hose clamp. You can use some weather profing around the top as a seal to prevent water running in through the split.
If you do take the shocks off, I think it is a good idea to fill them up. Might be a bit messy tho'.
In any case, take the opportunity to pump out all the old fluid, and replace with new. It's not possible to change all the fluid (what is inside the shocks) with a normal fill-and-bleed exchange.
Thanks for the idea of fitting an outer casing, will have to get under the car to measure the diameter.