I've never done this myself, but I heard somewhere that any "thin white wall" tire, like the ones you see on the beat ford taurus station wagons and such, can be shaved to have wide white walls. Like
Like I said, I dunno if this is right or not, but aparently all with those tires the whole sidewall is white, and they just cover it with a thin layer of black rubber.

Anyone got an old thin white wall tire lying around? Try it and tell us if it works!