If you just ask Larry, or Jr. Martin, they will be happy to send you a replacement spring for a nominal fee. It should arrive quickly because it is flat and can go in an ordinary letter envelope.
The springs I got from Jr. were not cut, so I could cut them to the exact size needed.
BTW, Jr. does not use safety pins. The last I heard, he bends and tempers his own springs.
The safety pin is a cool alternative and I'll look through my sewing stuff if I have any of the right size.
Yet I'll also email Greg Mouton, if he wants to send a couple they could come in handy.
If you know of a piano repairman or store that does piano repairs go there with your old spring and ask for a piece of piano wire of the same gauge. I used piano wire purchased from Schaff piano supply for making my bass springs.
I'm sure Greg will send you one ASAP. You should probably get one from him so it matches the tension of the other. They're pretty easy to put back in. Just gotta fit the ends in the tiny holes