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.[1][2][3] It is a design or scene in intaglio that appears “en grisaille” (in gray) tones.
Custom Lithophane Nightlights can be bought here:
https://www.etsy.com/listing/960554461/lithophane-night-light?ref=shop_home_active_7
or a regular custom Lithophane can be bought here: