John,
I have owned 2 broken springs (one was in a gun and the other I picked up as parts, I didn't break them!). The hooks were both good. They were snapped about 1/3-1/2 way down from the hook. One had the main spring (with the hook) broken and one the auxillary spring. I salvaged the good main and good aux spring and riveted them together...worked great.
I bought the disassembled spring in the photo that had been fastened together with a lousy makeshift rivet. The rivet had not been properly peened so the whole assembly was floppy. I will get around to putting a new rivet in it someday.
Once upon a time I needed an odd shaped flat spring for an old revolver. I didn't know diddly about steel types, but I found some steel shim stock at a machine shop that was the right thickness. It was in the white and cut relatively easliy so it must have been annealed. After cutting and bending it to shape and polishing it, I heated it to peacock blue and tried it. Got lucky the first time! This was an internal spring that worked the cylinder pawl so it did not have to be terribly exact in its performance. Just thought I would mention where I got the steel as a possible source.