You got a digital Caliper and are able to measure the width and thickness?
I have a stack of already tempered Spring Material here and can shove a couple pieces in the mail.
It's a little chewy to work with, but will file OK.
Another option (and likely a better one) is to hit up the Rabbi (he still around?)for a coil spring, and make a guide for it to substitute it for the flat mainspring, if at all possible. Worth looking at a single shot Cooey Shotgun or similar to pick up the gist of it.
Hmmm... A quick look around the interwebs and it looks like a pretty basic flat spring. About any spring that is the same (more or less) length width, and thickness, can be modified to work. For the bends, screw a stack of washers on to a board, place another screw to hook the spring around, and then push it around until it takes a bend you like. Won't hurt the spring. This is exactly how almost every spring out there is made these days, the material comes already hardened and tempered, and the springs are formed accounting for the spring back when they are shaped.
Rounding over the end to fit the notch in the butt is a piece of cake.
