Keel Shoe
If your talking about the fitting that the mast sits on mounted on top of the keel - I don't know that I'd use lead because lead is soft and the wall of the mast might cut through it. If you know who made your mast (it's probably an ISOMAST) you can buy one from RigRite. They have a web site you can search for mast parts by manufacturor. See the attached link to get to this site.