In west France you’ll find Betty Boibrut’s amazing little mobile sauna making the rounds of festivals. For a few Euro you can enjoy their hospitality and treat yourself to a massage from one of the attendants.
This is the only sauna mobil (or sauna ambulant) we know about in France. Although a saunaab was built in Farges by some homesick Finnish expats some years ago, but it has since returned to Finland.
They have produced a video to give you an idea of the experience.