Alternative to Escolar ("Butter Fish") for Nigirizushi
Escolar is not butterfish. It is considered to be more closely related to the Oilfish species.

Butterfishes are from the family Stromateidae, which does not include Escolar.

As far as I know, there are no digestive issues reported with butterfish. That "wax ester" is highly localized to Escolar and Oilfish. So if you're looking for a "substitute", just use actual butterfish. Source it from somewhere you trust, or look for a more specific label like Pomfret or Harvestfish.