It’s possible that the commercially available ear wires are alloys
designed for hardness, but you can harden your own wire, anyway.
When I’m making ear wires, or anything that needs a nice hard piece
of wire, I just stretch the wire a little. Most of the wire I have on
hand is soft to start out with, by necessity, so when I need a piece
to be stiff and springy, I lock one end in a vise or clamp of some
sort, grab the other end with pliers, and carefully extend it.
It’s a good idea to do this along the edge of a bench, or with some
other reference, if you want consistent results, because you can
easily monitor the change in length that way. I do it by feel,
having done it so much, but it is possible to get carried away… you
can try setting a target length as some small percentage of the
over-all length and put marks along the bench, or even lay a
yardstick or tape measure next to it.