The links I promised earlier, starting with vintage bouts:
Abziehsteine
And newly mined stones:
Natürlicher Wasser-Abziehstein - fein - Toolshop
I do not believe these are the Mueller stones that you see advertised from time to time. Those I have seen reviews on as being gritty and not as fine as a Thuri should be. These are Hebertz, maybe the quality is a little better from them than the competition?
I would probably buy from the seller in my first link though, given the exceptionally low price in the second I suspect a knife quality stone that may not be suitable for razors. But I have yet to buy one of either so again can not speak to quality of either. I'm fairly certain some users here have a stone from the first site though, as this is where I originally found the link to the seller's page. Maybe somebody who has one of his stones can vouch for him, but I suspect they are as advertised - good quality reclaimed material.