Sash spring

I'm sorry to say that we won't be able to help you with this project. The kind of springs you have here are built into the jamb liner and not typically sold separately. The jamb liner itself is effectively the balance, and that's what you'll need to replace.

Your best bet at this stage is to contact the original manufacturer directly. If you aren't sure who they are, or if they're out of business, your next step would be to contact a local mill work that still makes wood windows. They may be able to set you up with all new jamb liners to match your existing sashes.