The sail material used by most kit and ARS makers is usually dacron. It is about the same as Ceconite used to cover full scale aircraft. Sewing the stuff requires a master. Some of the dedicated racing types used molded and paneled sails. Take a look at www.amya.org for a list of sailmakers on the links. Most will make anything you want and they are knowledgeable about design and material. You will have to tell them what your yacht is and what sail plan is required, if known. Is it Marconi rig, mast head, gaff headed or a schooner ? Sails do not come sheap when made professionally but they usually last for a good long time and are probably correct as far as material and design.