Single Line, Sport or Stunt Kites all consist of a sail, spars and fittings.
Most are interchangeable by size.

The main advantage of using carbon graphite spars for your kite or project,
(vs. fiberglass),  is because Carbon spars are stronger but weigh less.
Single line kites usually have wood, solid fiberglass, or hollow carbon spars depending on the size & quality of the kite.
Sport kites, RC planes & boats usually use solid fiberglass or carbon graphite tubes.

Please refer to the diagram to locate the parts you need.

For A Quick Fix -
or if you are unsure what size you need, you can get wooden dowel rods at any hardware store.
Even on the beach, you can usually still fly your sport kite with a wooden dowel until you can get the correct spar. It will not be as responsive because it will be heavier, but it will fly.

How to Cut Spars -
To cut spars, You can use a Dremel motor with an abrasive cutting wheel
for the cleanest cut possible. Or, a fine tooth hacksaw can also be used.
Be sure the tube is held securely in place, and use a gentle touch so as not to crush. You can also tape around the area to be cut to prevent splintering or separation of fibers.