there are a LOT of custom spring shops that will MAKE you a spring to your spec's
a GOOD smith should be able to tell you what is needed or Get you a new spring
MY guess is, your smith doesn't think its worth HIS time, to do so, or doesn't want to charge you what he feels his time will be in this project!

this is the problem you can run into with custom built guns

do you KNOW The maker of the gun??
most LONG time gun smiths MIGHT have better ways of contacting him if you don';t have a way

try smiths in HIS area and ask away!,