If you have a Grandfather like mine, you might have a few (or a box full) older blades around that you can get for free. You'll get atleast two blanks out of a circular saw blade. I like old band saw or hacksaw blades (1.5" wide) best.
What ever you use, cut a small piece off, heat one end up to cherry red with a torch and dunk it in some oil (I use olive oil - cause that's what is in the cupboard, and it doesn't smell as bad as petroleum based oils. Watch out for the flare up) and let it cool down. Then see if a file will bite into the hardened edge. If it does, find another blade to try. If the file won't cut into the material, then you have decent blade material.