Brownell's or NEGC sells the mastodon ivory beads; I've used them and they work very well. As Dean said they can be stained to darken, and if they're too large I have chucked them in my Dremel and run them against fine sandpaper. They come with a shaft that may have to be cut down and trimmed a bit to fit the hole; I secure them with a drop of epoxy. Here's one on my Parker SBT; I didn't darken it because the original mid-bead was still quite white and I wanted them to match.
