The pull/pull cables must not stretch under any circumstances. The spider wire fishing line I found was unsuitable. There is a web site called "intothewind.com" for kite flyers. They have various types of Spectra line at a very reasonable price. You can get it in different strengths. I Use 50#. It comes in rolls of different lengths.
I buy the 50# in a 200' roll. You can build a lot of pull/pull setups with that much.