さゞれ銘砥(株)[330mate.com]

It's expensive, but quality is exceptional, worth every penny. I'm using this Atoma stone to flattening my japanese whetstones (artifitional ones, 1000...3000 grit), and it works like a magic. Highly recommended! Pay once, cry once, and will not regret it, believe me :) Just remember - when you are flattening your whetstones, they must be very wet, and your Atoma plate must be wet, too.