About the lightest thing you're going to find commercially available these days is the brand name "
Real Reef rock", which is made from crushed/powdered coral or other aragonite and binders.
There are lighter rocks out there, but they'll have been things like Fiji or Tonga or Marshall island live rock, and you're liable to pay through the nose for them
IF you can even find someone willing to part with them, since they've been banned for import/export for some time now. There's also Pukani, which is "dead rock", and about the lightest of the type you can get, but it's also now "discontinued" and quite rare as well.