First of all I want to point out that the home made fid is not somehthing that I personally came up with. It was shown to me by my friend/splicing mentor Max Underwood. My guess is that the design has been around for a long time.
And yes Norm it is piano wire, but you can get it at most any hobby shop listed as "music wire". There are many guages to choose from, the one in the picture is .039
It is incredibly cheap to purchase and lasts for quite a long time. I have also used that fid for doing used climbing lines (16 strand) because again the foot loop really helps with pulling the crossover.