There are certain levels to farm those three, but you can also get them on certain quests that appear on Friday, if you've unlocked Special already. You can also get various evolve medals (the fairies) through the week, with the higher tier (4 & 5 star evolve medals) on the weekend. These are all in the Special Quests group, which I'm pretty sure has to be unlocked by a certain mission.
Anyhow, the missions to farm those medals are 57 (You can sometimes get Merlins.), 80 (Cheshires), and I think 174 (Fairy Godmothers/ the rare Yen Sid).
Edit: There appears to be a Walkthrough pinned towards the top. Check one of the Medals things and I'm sure you'll find a better answer than mine.