Same here Joy - my two ex-barns are an absolute joy to have in the garden and they have not cross-infected any of my non vaccinated pure breeds. Sadly, I don't know of any organisations in France that could help you
Well I don't want to put people off rescuing ex batts which I think is an admirable act of charity but I do want them to do it with their eyes open. Ex batts aren't a cheap or happy option if health problem ensue so only get them if your over-riding wish is giving them some life rather than an early death.
Why do I say "cross infect"? Note not "infect" because the two groups of chickens come from different worlds and have different immune profiles. Batts live in a highly closeted environment of a laying shed where their immune systems have been exclusively taylored by man for the close confines they live in. They are reared on an all in, all out system and enter a sterile environment so they don't get exposed to pathegens from past flocks. Their health is maintaned by being vaccinated against all rapidly spread diseases they are likely to encounter and they are regularly medicated against infections and their environment kept as sterile as possible. Consequently their immune systems are suppressed and taylored by the medication they receive. Vaccines maybe de-activated or mild forms of live disease so that they may become carriers though immune themselves. Live vaccines are sometimes prefered because they are cheaper and give more rapid immunity. Since they won't infect their flock mates and succeeding generations under the all in/out system that's fine while ever they remain a closed community. But introduce them to the "free" world though rescue systems and this situation is uncontrolled.
Free range hobbyist pure breed chx are rarely vaccinated however they develop their immunity from the environment they are exposed to plus their contact with past generations. When you bring these two worlds together with their different immune profiles there's always the risk that they will
cross infect each other with infections neither has previously encounted and hence has immunity for. Any individual which is weak or sickly is a health risk to the entire flock and will be a source of incubating infection which can spread. This coupled with the fact ex batts immune systems may be damaged is why it is risky. The more ex batts the greater the risk. A couple of exbatts might not be much risk for example.
If you still want to take this risk it is best if you source commercial rescue chx which are from a free range system as they are more likely to have had some exposure to the same pathegens as your free range birds. To a lesser degree
any introductions, not just exbatts, carry a risk which is why most experts advise minimising introductions and keeping generations separated which is what I try to do. What risk you take is up to you and what I've described is not sure to result. Quarantining introductions for at least three weeks is a help as it should catch some latent illnesses, any stress related and some that the new environment might induce but it's not 100%. Keeping them in their own closed community is relatively safe.
Hope that explains my comments more fully.
HF