I take it that you have powerisers by that picture
You can replace one spring on them with a poweriser spring (or with a PJ one if you can figure out what weight you need
Alternatively you can replace both with new PJ springs (or poweriser ones)
Depends how old they are and how much use you have had from them
If you have used them for a long time it will be worth replacing both springs as the other one will be soft and not match the new one
If they are newish or haven't been used much you can get away with just replacing one
Jason