There are a lot of different string materials available. D97, FF, 452X, all will make a great string. There are videos on youtube showing how to make a Flemish twist string. There is also DVD called Doing The Twist. Once you figure it out it only takes a few minutes to make a nice bow string.
If you can make it to the Tennessee Classic you can buy spools of bow string material for a lot cheaper than you can buy it online.