There is something called "
Oasis Rootcubes," It's actually a foam even though it looks like rockwool cubes. I have not tried it yet, but probably will thee next time I need some cubes. I am sure it's sold in large blocks also. I'm also not sure how you intend to use it (just for seedlings etc..), or what type of system/s your planing.
P.S. Do you have a link to the "growing sponge by Grow-Tech" (with prices)?