Not sure where you are in France, but Johnc is in the North West area of France. He's rebuilt his Rapier and might be able to help with advice on suppliers over there.
I have a stainless steel exhaust on my SIV and did make some modifications to it but obtained the parts from a UK supplier and collected them on a visit to UK,so I can't help with suppliers in France.
Thanks folks.. have got local garages trying to find a small length of mild steel pipe (38cm o/s dia) to get us back on the road... then will try to replace whole system with s/steel later in the year.