public IEnumerable<T> FindAll<T>(DependencyObject rootObject) where T : class
{
    for (int i = 0; i < VisualTreeHelper.GetChildrenCount(rootObject); i++)
    {
        DependencyObject current = VisualTreeHelper.GetChild(rootObject, i);
 if (current.GetType() == typeof(T))
 {
     yield return current as T;
 }

 foreach (var child in FindAll<T>(current))
 {
     yield return child;
 }
    }
}

DependencyObject myUserControl = GetUserControlFromSomewhere();
foreach (var button in FindAll<Button>(myUserControl))
{
    // dem button irgendwas antun
}