Yes, an old leather belt. The type that is thick and usually used on jeans. Probably vegetable tanned like our favorite saddles. At least some of us. I haven't done it myself. I came across it on the internet before YouTube became the go to for that type of information.

Here it is: Estwing Hammer - Leather Handle Replacement

Edit: I looked it up and there are several videos on refinishing the stacked leather handles. Here is one:

Refinishing a stacked leather handle Estwing hammer

I have a preference for Estwing hammers. Mine are the blue rubber ones that while not as elegant as the leather, they are tough. Recently, I was using my grandfather's wooden handled carpenters hammer with only an oil finish and it felt so nice that I may have to change my affection.

What I really learned is that those hammer handles that I sanded and finished with urethane or shellac are going to have to get their finishes removed and all are going to be finished in boiled linseed oil or tung oil. No more film finishes on tool handles.