There are lots of thoee tires on eBay. A seller Bicyclebones sells the reproductions in sets and in singles. Also I think the Memory Lane Classics in Ohio still has some for sale.
Also, Bicyclebones has the best prices on ebay as for as Fastback repop tires goes. He has the good ones, not the cheap ones I have seen other guys try selling.
More praise for bicyclebones on eBay . I just bought a Schwinn Slik for a stingray I have and when it was aired up it had two small splits in the side wall that you could not see when it was not mounted and aired up . Totally not his fault and no way he could know but I contacted him and he told me to toss that tire and he would send a replacement right out which he did . I have since purchased two more tires for other Stingrays I have as a result of him being great to deal with .