No idea about the States, but over here Maplin among other suppliers of radio parts sell dial cord from the roll. The stringing diagram explains what length to start with, how many turns are required at each pulley, which end of each is on the top and where the springs go. It helps if you use a small spring balance to set the tension, though in this case the length is stated so when the route is right, then so will the spring tension.
Mistakeith, if you can't find dial cord in your location (unlikely as I'm sure that Tandy sell it and their "Radioshack" outlets are everywhere), PM me an address and I'll send you the right amount for the job. It's easy to do, anyone who can fit a set of Calvert blinds and get them meeting with the correct overlap can do this.