I.I.Systems 01-4783316 have everything needed for sash windows
Sash cord comes in 3 different sizes---6, 8 and 10 mm the size you need will depend on the size of pulley you have and the size of the hole in the top of the weights.
Try to go for a nylon cord because I have had to replace some cords my competitors have installed only 2 years after they put windows in