Cinderella is now almost always in Town Square Theater (used to be Expo Hall in Town Square) so if you miss her at Askershus that is the best place to find her... when we were there last month the lines were MUCH shorter than they used to be in Toon Town and even if they are long you can get fast passes.
As far as Fairy Godmother and the Step Sisters, they all can normally be met in Fantasyland behind the castle (best bet is the morning or early afternoon)... if you see any characters out in Fantasyland (Pinocchio, Peter Pan, Pooh, etc) if you ask their Character Attendant they may know the exact times... after about 10am you can also ask at City Hall and they can call the character department to ask when they will be out for you... actually just about any cast member with access to a phone (ie Merchandise locations, etc) can do that but not all of them will know what you are talking about. Sometimes you will get someone who will tell you to check the times guide and in that case you have to ask politely but firmly for them to call to ask the exact times for the exact characters you want.