yeah sprechners are force carb only. The only 2 natural carbonation ones are gnomes and some retro looking brand.
For yeast, honestly I've been using red star cooking yeast. The root beer after about 4 days carbed and was actually really good (I drank all relatively early so it was like the last 5 bottles). (gnome root beer, redstar yeast and brown sugar)
The last one was ginger beer and its pretty rancid, the only difference was I substituted the brown sugar for cane. I don't know if the yeast had eaten some taco bell or what, but when I pulled out my first test bottle from the fridge.... it smelled like someone had managed to fart in my soda.
Kind of turns me off of soda since I have about 2.5 gallons of spicy fart soda LOL
Got more root beer to make atleast.