A pillars of any kind are hard to come by. You might try Ebay , Moparts, Allpar site or go to the parting out post here or for sale post or parts wanted post.
Ok just went over to parts car section. On Feb 10 ,half way down first page , Chryslerfat has 65 Cuda parts for sale . I don't know if 65 Dart and Cuda are the same in that year. Send him a pm and see what happens. Good luck.