It is available in several different grades of powder grit size. I have bought it from Luthier supply houses but other places carry it.

Way back when, it was also recommended as a way to finish off plastic butt plates and grip cap edges which had been sanded down with the stock wood. Used wet on a felt or cloth pad.

The previous comment is correct. It is the same thing as pumice powder, just ground to a much finer mesh size.