I've come across the idea of bacta tanks in the extended universe, and my memory from the films is sketchy. As most of my reading about them has been from, well, back ground reading and not from any novels detailing them and that I don't think they're covered in detail in the film, I'd like to know, how common are they? Especially during the Great Jedi Purge?
Bacta was invented by the Vratix on the planet Thyferra in antiquity, with the galaxy at large learning of its properties around 4,100 BBY (src: The Essential Atlas). So by the time of ABY it became fairly widely used, though not 100% ubiquitous due to supply constraint.
Bacta was used for a while, but for a time shared the market with Kolto.
When Kolto supplies collapsed, Bacta became fully widely used.
Bacta was synthetic, but mostly produced on 1 planet, until the events of X-Wing novel "The Bacta War", when the Vratix finally exported the process off-planet.