All refined punga plants must be picked within the Ark & Dove cathedral (with one exception). Only one refined punga plant can be found outside of the cathedral or merchant inventories. It is located right beside the cathedral gate.
After completing the side quest Tailing the Tomboy, your reward will be 10 refined punga fruits.
It can be bought from Capital Wasteland merchants after Point Lookout has loaded, they will even restock faster then it takes for the ferry to reach Point Lookout.