I've been to dozens of sites in France with fishing but not in August unfortunately which may be completely different to what you find in May/June or September. Usually (and unbelievably) the fishing is free and very often I have the whole of the fishing to myself, I've only been charged twice and that was on British owned sites, (but maybe the French charge in August). Avoid rivers and river fed lakes unless you want to pay for a French fishing licence, approx 30 euros for 2 weeks I think.
Some good ones :-
Chateau de Chigy near Luzy, Burgundy (ACSI) - Several huge lake, up to 25 acres, planty of carp. Was the only fisherman there!
Vielle Chene near Dol de Bretagne, Brittany. Loaded with carp, choice of lakes.
De L'Etang near Brissac Quince, Loire. Smallish lake, restricted fishing pegs, superb fishing if you can avoid the small catfish.
It is hard to find bad ones however, even sites that don't advertise fishing often have small lakes crammed with starving fish.