Window spring balances

Hi, I'm working on some older wood, tilting doublehungs. The balance system is vinyl sash liners with foam backing clipped to the wood frame. A metal bar at the top of the sash liner supports a pulley. The balances are naked springs that are clipped to the sash liner down low. They have a cord that runs up through the pulley and back down to a stud on the window. The sash pivots on these windows are approximately in the midle of the sash, vertically. The balance springs are about 24" long. They failed where they attached to the sash liner (rusted out). I'd like to find replacement springs. Sorry about the wall of text!

Thanks,
Eric