Another good source for hammer stock is IHCP bar, aka hydraulic push rod. IHCP stands for induction hardened chrome plated. It is 1045/1050 steel. It is found nearly everywhere. Most, if not all, of the chrome plated bars that you see as part of a hydraulic system are IHCP bar. You know the chrome plated bars you can see that do the pushing and pulling on bull dozers? That's them. The bar that lifts up a dump truck bed, etc, etc, etc, that's them. I've made several hammers from this stuff.
Now safety trolls you may have your field day telling me how dangerous the chrome MIGHT be. As long as you have good ventilation, it should be no problem. Life is short. I would rather have a sweet hammer than worry about chrome every now and then. I will admit, it would be different if you were making several hammers per week, I buy my hammer stock new from a supplier now.