you don't want the rock wool.
Cocogro's web site isn't explicit about what the actual material in made of. they also sell (composted?) ground coconut shells, but from their their description and the descriptions given by their distributors i would say that their product is aged coir fiber that has been chopped to a ~3/4" length, aged outdoors, and then washed to remove the fines and salts.
this would probably be a better material than the raw coir bricks which you should be able to find at The Home Depot or Lowe's, which i would guess are "new" fiber. I've only seen coir in "bricks" which are about 4" x 8" x 8"; my local store has several variants for different purposes (orchid mixes, cacti, seedlings, general use) but as far as i could tell the bricks were identical -- only the shrinkwarp wrapper changed.
$11 / 5 kg online is probably about what you would pay locally for 5 kg worth of bricks