I ordered the late Kit Ravenshears booklet on making V springs from TOTW and he recommended using the spring steel from DGW as it was the most forgiving in regards to heating and tempering. I think it's 1074 or 1075. it comes 12" long X 1" wide and they carry 5 different thicknesses, 1/32", 3/64", 1/16", 3/32", and 1/8". $3.75 ea. plus shipping. You can order just one piece if you want and they won't have a problem sending it to you. large order or small order, it doesn't matter to them. I ordered some a few months back and I'm getting ready to make a mainspring for my Swivel Breech project with it.
Capt. David