The best site I have found for Hones and Naniwa hones is the following:
Japanese Waterstones: Naniwa
You have to page down to see the hones.
But clearly there is a 12000 grit.
I've just ordered the 800/5000 to complement my Chosera. I'll let you know how it goes but the 800/5000 and the 3000/8000 combo stones represent great value. The price will I think probably rise soon.
Interestingly on the Naniwa Duetschland web site, they don't list the 12000 hone or the combination hones.
Also to add to the confusion, they advertise a Superstone range and a more expensive Superstone + range.
So it gets more confusing.