It's hard to believe that Muzan lived for over a thousand years but never found an existing flower. Even if that flower only blooms under the sun for a few minutes, one of his many human acquaintances should've found it at least once.
(It is said that Tanjiro's mom, Kie, found the flower and showed it to him once, it was in the same place where Yoriichi buried his wife, Uta).
When he was impersonating a child in the manga, the human family was more than ready to find a cure for his 'sickness'. He could just say 'I need a Blue Spider Lily to make a cure' and the family would probably comb all of Japan to find it for him.
The idea that Muzan was never able to find the flower because the Blue Spider Lily was a flower that a genius, but generous, doctor personally cultivated to help people seemed much more logical since Muzan killed the doctor, the only one who was supposed to know how to cultivate the flower.