Janie,
I've seen some on flebay, and I actually accidentally found one at my LSMG. But in lieu of that, they can be made easily. I made two from old fashioned skeleton keys. Just chop the end off and file the new end to fit the locks. I have three treadles and on bent wood case and they all take simple rectangle shaped key ends.
If nobody else has answered you by the time I get back I'll post some pics of my home made ones.
Joe