Of course most actual "music/piano wire" you see in the hobby market isn't suitable for use as actual piano strings...but it's the same basic stuff just not quite to the same quality constraints.
Basically it's a stiff high carbon drawn steel wire.
I just use MIG welding wire. Very similar, easy to find, but it comes in spools so it has a slight curve to it. I always use coffee stirrer straws to ensure that the wire doesn't bend/flex.
I always take the opportunity in threads such as this to mention a bundle of marking flags at home depot or lowes is cheap as beans and unless you need a superbly long control rod (in which case i would be looking at carbon rod) they work amazingly well.