Toilets, Sinks, Showers, Tubs and Disposals - drain pipe broke in sink flange

I was removing the "J"(resevior) from under my bathroom sink. The piece of pipe that runs from the flange to the "j" broke off completely. A piece of the broken pipe's threads are still in the threaded area of the nut on the underside of the sink. I have no way to get a hold on the piece of broken threads. I was hoping to replace just the nut and the broken pipe. When I called several places, they told me they didn't have just the nut, and that I would have to buy the entire drainstop assembly.
Its an old sink, and the drain stop assembly is entirely inside the porcelen part of the sink. I haven't seen any drainstop assemblies that would fit my sink.