I'll try and find something more useful tomorrow, but for now, as ever, the best advice is, when you get there, to go to a book shop and buy the local map in the IGN's blue 1:25000 series. Dolmen's are picnic tables and menhirs are triangles. It's usually fairly pot luck what's there when you arrive.

Ok, mid post i've found this: http://prehist.free.fr/ Bruno Marc is the man, find his books when you get there. The best thing about Euro-megalith finding is you never know what you're going to get...

If I think of anything else I'll post more...