Hello Laura.
(I'm assuming you're not far away from Toulouse. That may be interesting also for other people in southern France.)
There is a little producer of 100% pure natural hevea latex mattress (and pillows too) near Perpignan :
Sommeil Nature
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Matelas 100 % latex naturel | Sommeil Nature
(Their previous name is DPLH :
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Matelas BIO | D.P.L.H. Les Déhoussables en Pur Latex d'Hévéa )
I met them in an eco-fair, years ago, then they told me that for
most of the "100% latex" mattress sold in France, in fact the hevea latex (produced, for instance, in Brazil) was added with a chemical solvant, and then carried from Brazil to France in tanks (in the liquid form), and then the mattress are made. So "100% pure" is generally a lie.
This small producer/seller import "solid" mattress already made (in Brazil probably), but they put themselves the cover, etc.
According to their website,
their mattress are made of 96% hevea latex + 4% zinc oxyde :
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Fabrication | Sommeil Nature
(Of course, one will have to check it with them.)
My two euro-cents.