Replacement grips are hard to find. You can buy a cheap hammer for less than a pen. But if you have a favorite hammer, a new handle will do. They must be tailored to the individual head, but must add years to life. If you've never changed grips, you should put your head down, hold your head down, take another hammer, and keep hitting the end of the grip. The head will bounce harder and harder. Trim the excess off the top of the handle and hammer in the wedges.