Measure the inside diameter of the tubes. Most non-Rev sport kites are built around arrow shaft spin off tubes, thus they have an internal diameter of 0.244 inches. Rev tubes (rods) have a slightly different internal diameter since they make their own product. The carbon fiber arrow shafts are made by wrapping carbon fiber cloth around a steel mandrel, curing the epoxy, removing the mandrel and dressing up the final product to length. The outside diameter is an indication of the stiffness of the tube (other, non-measurable variables will affect stiffness also) and will help you decide on a replacement tube.
With inner diameter, outer diameter and length in hand, look at this website to see if anything comes close. Look under the P-series tubes first since they're the most commonly used. If that fails, then call Jon at Skyburner and ask for his help ... he's an extremely knowledgeable, friendly and helpful individual.
Cheers,
Tom