Do you have snapped cords? We often replace just the sash cords. We use a UK manufactured waxed cotton sash cord with a nylon centre. This makes it tough and durable but it still looks period correct.
We work with many landlords in their rental properties where it is vital to keep the windows operational but on a budget.
A re-cord is part of our standard window restoration. However, if you do not need a full restoration this service may be a better option for you.