I have ordered 2 plants through my local nursery, it is hard to come by. I can only order the plants within Western Australia, because of our quarantine laws. I'm going to try growing it in pots to begin with.
I use stevia a lot, but have only been able to get the powder. The fresh leaves, I'm told, have a better taste.
I couldn't find anywhere that sold the seeds, but you might find the seeds in your state.