We went a few years ago and used Latesail but I see they don't have anything available in that area at the moment. However, a quick Google search on "Port Crouesty / Port Navalo" (the main charter bases, I think) gave this link which may help: http://www.crouesty.com/activites.php?langue=uk&smenu=SPL&activite=LBAT
Yes - Nautilus do it through Spi-West in La Trinite. Warning - take your own bedding & towels though, we were given absolutely miniscule pillows and towels! It appears they are used to the French bringing their own, so they threatened to charge us a small fortune 120 euro or so - each - just for bedding!
Good boat though (Ovni 385) and great cruising area. I would recommend Ile Houat for anchoring and a run ashore. Be very sure about the tides, though, when venturing into the Morbihan. They can run at 7kn or so in the entrance!