There is a fisherman that sells from his cooler in front of the fisherman's boat ramp in Cruz Bay; that is off the same parking lot as the Customs building. Look for a white cooler and a scale hanging from a tree.
Homemade Meals to Go in Coral Bay sells fresh fish from Puerto Rico that has been butchered, vacuum packed, and frozen. Calabash Market sells fresh fish from a local Coral Bay fisherman. See the Coral Bay Cruising Guide for locations -
https://www.cbycstj.org/provisions-restaurants-retailCheers, RickG