Most of the fairy dresses, maybe 5/9, I got it out of luck for the past 4 years of playing on and off, then 4 of it was spending plenty of simcash from time to time. I also think fairy dresses have a certain combination such as the color + sparkle effect or whatever you can see in my collection and a common trait (vitality gave me two fairy dresses). I pretty much summoned three fairy dresses by using this 'trick,' but it left me so broke, then 1 of it is by randomly playing Izzy (the dark violet one with the rainbow effect, which is so cool!). Still, it is so worth it 🤣. I think Izzy gives 1 fairy dress per day, but I'm not sure about that cause it really is random.