The trouble with all the French harbours on the N coast is that they are half-tide or weather-restricted harbours, excepting le Havre, Dieppe and Cherbourg.
I don't know of any web-site but Bloc Livre en Bord will give you every detail, including prices, max berth lengths etc. In France it's only FF99 and I believe Imrays handle it in the UK (01480 462114).
If you're seriously thinking of berthing over there go round the corner to Brest, or somewhere in S Brittany.
The sailing, weather and ports are much better round there and prices (away from the infection of MDL about 25% cheaper than the N Coast).
The farther S you go the better the prices £25/month for full pontoon facilities for a 10m boat is not unknown in the Vendée.