i'd start with welding stock from Lowes. years ago i made a sword on a bench grinder out of the stuff. won't hold an edge as-is, but it'll make a nice sound, and you can put it back into shape fairly easily if/when it gets warped. i think i paid about eight dollars for the piece i used.
if these are just for a play, why not use aluminum? you can always pipe in clanging sounds through the speakers.