You should be able to find a quality thuringian with no problem then

The great thing about learning with synthetics is that you will build a honing foundation on "known" stones, like the Naniwa 12K. If you ask a question about a 12k, then everyone will be giving advice on the "same" stone. Not so with naturals as they can be all over the place, this has been my experience with coticules. Once you have established that honing foundation, you will have the knowledge to delve into the naturals and will have an idea on where to start, what "feel" is all about, and you will be able to estimate the "grit" to know if you have a finisher or pre-polisher in your hands.