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Troc-vélo is the main specialised marketplace. Better chance to find a good one on there than LBC, but you may have to spend a bit more. Saying that there's a Follis for 200€ atm which doesn't seem outrageous to me.
It's also worth contacting busy bike rental businesses, at least the good ones, as they change their fleet often. Last time I rented a tandem I was told by the guy there that they sold their bikes every year but they had a waiting list for their Cannondales tandems that was several years long.
Where can one source a tandem in France? Asking for a friend.