Blizzard is addressing one of the biggest player complaints with the massive character list on World of Warcraft’s login screen. In Patch 11.0.5, they are improving the search function, allowing you to search your characters by Realm name and Specialization.
Simply, type out the Specialization name in the search bar above the Favorites List and you will see all characters with that spec.
In a similar vein, typing out the Realm name will show you all the characters created on that realm.