Order a piece of W1 or O1 from Brownells. It comes in rounds or flats and is quite soft until you heat treat it. A truck or car lief spring will work fine but you'll have to anneal it before you can file or cut it. Your chisel doesn't have to be thin you just want to cut a thin piece and then bend it back where you want it. Hold it in a vice and use the vice jaw as a guide. Spring making takes a little know how. Read up on it. Then make a practice piece or two.