Where to eat John Dory fish dinner in Paris?
Ptipois is right. I know of no restaurant that has it permanently. All self-respecting restaurants only have it - and all other fish - selon arrivage. If a restaurant has it on the menu permanently, it will probably mean it is not fresh.
St Pierre is also one of fave fish. I also choose it whenever I see it on the menu.
Depending on your location, I would consider L'Acajou in the 16th. I had the best John Dory (prepared similar to Sole Meuniere) I have ever had there. On the other hand, I had a terrible meal at Le Dome (the seafood platter and the Sole were both average to poor and not freshly prepared.)
It can show up on any menu, it is not particularly a frequent occurrence but it does show up sometimes. It is good, with a firm and dense flesh. The best way to have it is grilled whole on charcoal or wood embers, as you find it at seacoast restaurants (particularly by the Mediterranean).
In Paris, your best chances of encountering saint-pierre (that's the name) will be the restaurants specializing in fish: La Cagouille, Le Dôme, Le Bistrot du Dôme, Le Duc, Le Divellec, La Méditerranée, Les Fables de La Fontaine, Marius et Janette, La Marée, Le Laumière, Vin et Marée, Dessirier, Goumard, Le Bar à Huîtres, etc.
(Avoid La Criée by all means.)