Waterline Systems in Portsmouth, RI should sell them. If you are trying to save a few bucks, see if you can track down Roberto Authier, he is the J Boats builder in Argentina. He will sell you a set, but shipping could eat up any savings.
I went with the Waterline setup as they not only have the right lengths but spiffy end caps. Pricing all of it out came down to a single screw up. One mistake will eat whatever you'd save. And agree on freight - it's oversized and not cheap to do.