The stuff I mentioned at Sew-Classic is basically what the hardware stores sell as lamp cord.
There is two thicknesses of insulation. SPT-1, the thin stuff, and SPT-2, the thicker stuff. If you get 18-2 wire it's the same in both cords, only the insulation is thicker.
18-2 SPT-2 is hard to find locally. I've yet to find it in Ft Wayne, IN. There is an electrical shop in Springfield, IL that stocks it, but so far not here. That's why I mentioned Sew-Classic.
Joe