I couldn't find spring steel shim stock when I had mine laser cut, so I ended up using 4130 alloy. I didn't realize that it was annealed, so my paddles bent at first after just a few mags. Apparently most shim stock is annealed to make it very ductile for bending into other shapes. Then I brought them to a heat treatment place and told them to try to "make them into springs" (I am no metallurgist)...they heat treated and tempered the paddles to Rockwell 38-42 hardness. I have not had any problems (still on my original paddle), but I also don't shoot a ton either.
I don't know what alloy makes the best spring material. Though, I'm not sure if the paddles really need to be that 'springy' anyhow. As long as they don't bend or break they will probably work just fine and not damage anything. From what the heat treatment people told me, I didn't want them any harder or they would possibly start to become brittle. I let them decide what to do to balance modulus, toughness, and hardness based on the arbitrary alloy that I had made them out of.
I also did not parkerize or blue them, so some of them have a little flash rust on spots. I didn't feel like that was necessary for what is supposed to be a consumable part. If you got one shipped to you and want to make it look nice, I'd recommend taking a small file and debur the edges where there were tiny tabs holding them in place on the sheet, sand or emery cloth the surfaces, and then use some cold blue to finish it.
Esox