Wish I could help you Anna. My original had been replaced with a plastic one and I discarded it and did not replace my water tank. There is a thread here about replacing the water fill. I'll see if I can find it for you.
I don't think you are going to have much luck finding just the lid if it's the flip style (that's what mine had, oops maybe I should have kept it). There is a thread about replacing the fill inlet and there was someone selling the vintage filler inlets back in Oct.
I almost said if anyone can find it, LadyW can. Mine was that kind but it was plastic. One of the few things not original on mine. That style looks good, a little sleeker than the screw on kind; the screw on kind gives you a good seal.