Dont know of a French wildcamping site but you can stay almost anywhere so long as you are not actually camping (awning ,tables etc out).
Just returned from a 12 day hop down the Normandy coast with Polly and Izzey in tow (she had not been to France on her own).We used aires all the time , Gravelines (free) called at Le Crotey for lunch but found the aire a bit grotty and 5euro,Saint Valery sur Somme, 8 euro barriered and a 10 min stroll into village,Le Treport ,stopped for lunch but didnt use the aire,St Valery en Caux,5 euros with sea view.Fe'port nice spot facing marina and free(Benedictine distillery for the drinkers),Honfluer 9 euro's with leccy if you can get it (we did)stopped here 2 nights ,Arromanches free in central village, Deauville/Trouville free aire 5 mins to seaside Rouen ,still possible to park on banks of Seine overlooking town on Quai Jean Moulin only stopped for the day as have overnighted before and its quite noisy traffic wise,The aire at Rouen is at Oissel to the south another free aire and they have doubled the camping car spaces to 4 but beware if you are over 6m the guy facing the aire will be out complaining you are not within the parking lines,and finally Fort-Mahon-Plage amongst the sand dunes 2 mins to the beach but now 8euro a night. We spent 50 euro on aires and could have used different aires that were free but as had no itinery we just stopped in the places that we like the look of.