I think music wire can be softened and re-hardened again. Somewhere there is a thread on that and I have no clue what keyword to use - but its here at RCU - that is your answer to a custom spring.

More coils or thinner wire equals softer spring.

Sorry this is all I have in my head for springs, but good luck.

You might want to look at RC Car springs at the hobby shop. Usually there are many many different shock spring sizes and weight. Maybe you can straighten the sections that you need from one of those possibly.