Hummer Lady,
I've searched all over the US and haven't found any belt shifter springs. I found one source in Australia.
http://www.annebonnyslocker.com.au/ckp/a.main/index.htm
The gals name is Cyndy Kitt. She sells the springs for $2.00 AUD + shipping.
I had a 1919 treadle with no spring and one from 1924 I was returning to a treadle after it was electrified some eons ago. Couldn't find them here, so I ordered them from her.
Joe