I make them out of plastic on a 3d printer and sell them at www.3dfantasyminiatures.com
From Wikipedia: A lithophane (French: lithophanie) is an etched or molded artwork in very thin translucent porcelain that can be seen clearly only when back lit with a light source. It is a design or scene in intaglio that appears “en grisaille” (in gray) tones.
Custom Lithophane Nightlights can be bought here:
or a regular custom Lithophane can be bought here: