Amazon.com : Ackerman Pens Super Sketch Piston Filler Demonstrator Fountain Pen, uses any ink, slip nibs and feeds in and out *note: pen will be dark gray, not black as pictured :
SAILOR 1911 Standard (Mid size) Demonstrator Fountain Pen - Transparent | PenSachi - Japan Limited Fountain Pen
Conklin All American Fountain Pen - Demonstrator – Paper and Grace