Способ соритровки коллекции ObservableCollection
...
ObservableCollection<string> _animals = new ObservableCollection<string>()
{ "Cat", "Dog", "Bear", "Lion", "Mouse",
"Horse", "Rat", "Elephant", "Kangaroo", "Lizard",
"Snake", "Frog", "Fish", "Butterfly", "Human",
"Cow", "Bumble Bee"};
_animals = new ObservableCollection<string>(_animals.OrderBy(i => i));
...
Вот и всё!