Until I got B-50 from Amazon, I tried lots of different materials for string, and found that the absolute worst was cotton, followed by hemp. My favorite material was 'kite string', but it stretched like crazy. I found that upholstery thread works great - it's very string and incredibly thin, and easy to find. Any fabric / sewing store will have it. It does tear up your fingers, though.
In the end, I'm so happy that I got B-50. It even came pre-waxed.