I think they stock this at Mr Bricolage (they do according to the website), but I haven't been for a long time since their prices are so high. They'll also have it at Aubusson Bricolage (in Aubusson) and at Bricomarché (in La Chatre).
Thank you Dave. As it happens we went to Mr Bricolage this afternoon and got just what we wanted. They have a selection but not in any great quantities. We bought some pine plinth which we can stain to match our floorboards
Kind regards Susan