Hi Mike - try the websites www.campingcheque.co.uk and www.campingcard.com for details of sites which offer discounts on the normal site tariffs during low season (September is definitely low season). In fact you should beware that many French sites close in early to mid-September, and facilities may be reduced on some of those which are open.
Both websites have an easy search facility to find sites, but search under Poitou-Charente as well as Loire Valley. The dates when Camping Cheques are accepted are oddly in the American format (I have made them aware of this - it's a new website, still with some glitches) but hard to get your head around when there's a list of dates!
Le Chant d'Oiseau (http://www.loire-gites.com/) is not part of either of the above schemes, is British owned, and does come highly recommended on another camping forum - haven't been there myself.
Almost every French town will have a municipal site, often centrally situated, so if you fancy a particular town just google 'Camping Muncipal.............)
In September you shouldn't need to book anywhere at all. We've holidayed in late August/early September for many years, and never book anything, we just go where the weather forecast is best.