thingiverse.com, or something looking like it, with a "click here to order on the theoretical future Amazon print on demand service" right next to the existing "Download This Thing" button.
Or if you must insert friction for legal reasons, simply download from thingiverse and upload to Amazon. Two steps not too awful.
On https://pinshape.com we give you the choice of downloading the file for your own printer or ordering it as a professionally printed design in over 35 materials. We're like a mashup of Thingiverse, Shapeways and Dribbble, since we also curate our designers and verify print-ability of each design.
Or if you must insert friction for legal reasons, simply download from thingiverse and upload to Amazon. Two steps not too awful.