Files make super knives, they have to be harder than the mild steel they are designed to cut! Another good source, (if they are still in use) are power hack-saw blades, about 3mm thick, 25mm or so wide and they come anywhere from 30cm 90cm long. They were used in machine shops. When they got dull, they were thrown into the scrap bin.

My Father made a set of kitchen knives for my mother back in the 20's from these, one of my brother's still has them, his wife jokes if they ever get divorced, she gets to keep the knives.

Just remember when grinding them to shape, not to let them get too hot, or you can draw the temper out of them, and re-tempering is a mystic art, practiced by secretive persons in leather aprons in dim caverns back in the hills.