There are several "plastics," both thermoset and thermoplastics, that are commonly used to simulate ivory. The thermoplastics are likely to "creep" too much under even minimal load, but one of the harder thermoset varieties could probably be used to make a serviceable peg. My recollection is that one I've seen is/was sold under the trade name "Ivoroid" or something like that. The problem here is that purchasing raw stock - even if you could find a supplier - in the small quantity needed for a peg, or set of pegs, would probably be prohibitively expensive, so you'd want to look for something a little bigger than a peg that's made out of an appropriate looking material and that's expendable. Is it bad luck to slice up an old buddha?

The "practical" approach would probably be just to use the best hard white wood you can locate, and find someone willing to experiment for/with you.

You might also try just painting a peg white; but most paint, even stuff like the epoxy enamels, tends to be "gummy" under contact loads. A possible "paint" that might work, in a very thin layer, is the "porcelain patch" stuff that's used for hiding scratches in your sink. It dries very hard and could perhaps be sanded to a good surface if applied in thin - and even enough - layers.

John