None of those statues have survived, even in part, which is too bad, really. I suspect that fragments of the bronze Colossus of Rhodes may still be found on the sea floor, although those have indeed been looked for. Apparently there’s a fairly current effort (again) to build a new C of R.
My wild guess is, the use of ivory on the large statues was limited, and a faux ivory made by some chemical process was used.
http://www.geopolymer.org/science/introduction