I've not seen them either, since the '80s when I worked for an auto parts store and could look at everything hands-on. I know GM performance and Accel both sold SOLID copper core wires, not coiled or varnished, with the disclaimer of not for "HEI, rev limiters, or similar devices" due to potential interference. Copper with nickel added does not fracture as easily, but I doubt they bothered. These wires were on the $ low end.