What is this sink drain bolt called?
Any bolt of the correct thread pitch would work. I'd suggest taking it to your local hardware store and browsing the selection until you find one that matches. I would recommend stainless steel so it doesn't rust and/or get nasty looking, brass would be a good alternative, though it wouldn't match in terms of color. Galvanized steel would be a second place. Bare steel would be a very distant 3rd, and I'd invest in a box of them so you can replace them when they start to look nasty.