Good job, Harvey! BMJR does, indeed, carry Polyspan. As does FAI Model Supply. Go to
http://www.faimodelsupply.com/starline-polyspan.htm
Polyspan is by far my favorite covering material for models weighing less than 4 pounds or so. For heavier models, like Large Scale sailplanes and gassies, I prefer Sig Koverall (because I'm cheap and so is Koverall!)