if(tbSearch.Text == europe.England.Content.ToString())
            {
                europe.Show();
                europe.England.IsChecked = true;
            }
            else if(tbSearch.Text == europe.Island.Content.ToString())
            {
                europe.Show();
                europe.Island.IsChecked = true;
            }
            else if(tbSearch.Text == europe.Nederland.Content.ToString())
            {
                europe.Show();
                europe.Nederland.IsChecked = true;
            }
            else if(tbSearch.Text == europe.Germany.Content.ToString())
            {
                europe.Show();
                europe.Germany.IsChecked = true;
            }

var searchText = tbSearch.Text;

var radioButton = europe.Items.Cast<ListBoxItem>().Select(item => item.Content as RadioButton).FirstOrDefault(item => item?.Content.ToString() == searchText);
if (radioButton != null)
{
    europe.Show(); // откуда у ListBox'a метод .Show()?
    radioButton.IsChecked = true;
}