I own a 3D Printer. You first have to 3D scan your Impressions, and it has to be really accurrate.
Then theoretically you could design your Retainers.
BUT, i dont think it would be comfortable to wear, because the surface of your Prints arent smooth. I dont know if the smoothness of the surface of a normal retainer is recreateble with a 3D Printer.
The next problem are the wires. I can´t imagine a way to integrate those, using a normal printer.
I guess its easier to create retainers at home using the normal way of doing it, than to Print it.