Braided Kevlar shock cord is incredibly strong for its weight and is heat resistant, so it makes a great shock cord in model rockets. Low power rockets can use 100 lb. tensile strength, while mid power rockets will do better with 250 to 750 lb tensile strength. High power rockets do best with 1500 lb tensile strength. Price is by the foot; just indicate how many feet you need in the quantity field.
A minimum of 6 feet of shock cord is best for small rockets; larger rockets likely need more--up to 20 ft--to ensure proper deployment of the recovery device. Longer shock cords also provide more shock absorbancy, though Kevlar cord stretches much less than elastic shock cords. Many times, this will help reduce damage to body tubes from the nose cone bouncing back and hitting the body tube, but the sharper yank at deployment requires a sturdy shock cord attachments. Can be combined with elastic shock cord to add shock resistance in heavier models. Just put the Kevlar closer to the motor so you benefit from its flame resistance, and add a length of elastic shock cord near the recovery device.