I recall reading somewhere there is either a Farmer's Market or a wholesale market on the island open to the public? Any leads on this?

Perhaps you are thinking of the Rasta Farm? It is on the right hand side of the road from Cole Bay to Marigot - look for an open gate and a large Jamaican flag(?), follow the road through the field until you reach the buildings. Closed Saturdays. What they have depends on what is ready to harvest.

There also a couple of fruit/veg stands in Marigot market, but mostly fruit and spices (no green veg).

Is that north or south of the bridge from airport?

North.

There are produce stands beside the main road going through French Quarter.