Choose a random subclass of a class

Suppose I have a class Letter and a lot of classes A, B, ..., all of them inherit from Letter.

How can I choose a random child of Letter?

I know it sounds a bit broad, but what I want is something like this

public Letter GetRandomLetter(){    //return a random instance of A, B or any child class}

The number of child classes is unknown so I can't just use some logic with random numbers to return an instance of each child class. I also don't want to use reflection.