I had a few extra sockets on a string I bought, but with the sockets empty there was an electric shock hazard from the exposed contacts, so I adapted this model.

This is simply the screw base from sat26's lightbulb model on Thingiverse, with an added notch so you can screw it in to a socket. 

Obviously you should never use metal-infused or other electrically conductive filament for this model.