Your best bet is to make one. It is possible to make a jig and bend a handle to suite, or you can look for a naturally curved branch.
I have used hickory to good effect. Northern framers have easier access to ash. Both woods seem to work fine.
Shape the handle as you want, using a drawkife and spokeshave. I shaped the main part of the handle, leaving end that goes in the eye of the ax rectangular. Boil the eye end until it is pliable, clamp the rectangular end to a stout timber, and the wedge the handle upwards to the curve you want. Let it dry well (months) and then shape to fit the eye of the ax.
