I doubt someone of Prince's means needed to buy street Cartel versions of Fen. He probably had doctors lined up offering him whatever he needed, legit(sic). Kind of like Michael Jackson.
Side note, my wife recently went to the hospital and they gave her fentanyl in a very tiny dose and its effects were so bad, we are requesting she never get it again. Just the small amount they gave her put her into an anxiety attack, feeling completely out of control, and couldn't stay still for a MRI. Completely out of her nature. I would hate to see what larger doses will do.