The best replacement for rockwool is fiberglass.

The best alternatives to rockwool are rated as following:
★★★★★  fiberglass
★★★  fiberglass insulation
★★  hydroton
★★  soil
★★  coco
★★  mineral wool
★  asbestos
★  703
★  perlite
★  glass wool
★  celotex
★  cellulose
★  coco coir
★  earthwool
★  rapid rooters
★  roxul
★  kingspan
★  polystyrene
★  oasis cubes
  fibreglass insulation
  polystyrene insulation
  cellulose insulation
  vermiculite
  root riot
  peat pellets
  owens corning 703
  glasswool
  rapid rooter
  knauf
  coir
  stg

Your alternative for rockwool is not listed? Suggest!
Speaking about rockwool, one probably shouldn't forget about hydroton.

Other search terms related to rockwool and their ratings are:
★★★★★ hydroton
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Google Trends

Scale is based on the average worldwide traffic of rockwool in all years. Click on graph for more.
rockwool
fiberglass
fiberglass insulation