Pull the frame and check the back of it for a moulding giving the manufacturer's name. That particular plastic style is cranked out by the thousands and there are a couple of major companies who make them to order, typically for dealerships. You may get lucky and be able to track them down; they may still have the original moulds.
Even if they don't, 3D printing these likely wouldn't be time- or cost-effective. Just send the company the artwork, let them set it up, and do a run. They'll probably want a minimum order, for the hassle involved in running off even a half-dozen on a 3D printer, it's probably worth it - not to mention that whatever you use needs to be at least as UV, weather, and vibration-resistant as the plastics the custom licence plate frame places are.