+1 on what amphirion said.
You can also purchase online at Aquarium Design Group
(a TPT sponsor) or Aqua Forest Aquarium
. But, your best deal will be through the TPT member sales.
You might also check to see if you have a local planted aquarium hobbyist group that might have a local sales board. And once in a blue moon, it's possible to find these items on craigslist. But, most people who have these kinds of high end stone don't usually sell on CL.