I work buffalo horn, since I make powder horns. I even have some here. How big would you need? I have one horn tip sitting here, and one I could take off for you (first step to making a powder horn, you know.) Either I could send you the horn tips, or I could attach them for you.
BTW, have fun dyeing antler. You could try boiling it to open up pores, but I don't think you'd get black. Just trying to give antler an aged look liked to killed me trying. But, buying buffalo horn like they mentioned would work just the same as what I have. It works pretty easily with files and sandpaper. Just don't breathe the dust unless you fancy lung infections.