Get selected items from a list box with check boxes

A have a list box with check boxes (I removed the part about alignment, width, margin as not related to the case):

<ListBox  
ItemsSource ="{Binding SItemCollection}"     
<ListBox.ItemTemplate>
    <DataTemplate>
        <CheckBox Content="{Binding Path=Item.Code}" IsChecked="{Binding IsChecked}"/>
    </DataTemplate>
</ListBox.ItemTemplate>

I have a class SItem inside my ViewModel, which stores two fields - CachedStr which I get from Cache and a Boolean IsChecked which represents whether the item is checked or not (CachedStr object also has several fields (Name, Code etc), I've chosen to show the Code):

public class SItem : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;
        public CachedStr Item { get; set; }
        private bool _isChecked;
        public bool IsChecked
        {
            get { return _isChecked; }
            set
            {
                _isChecked = value;
                NotifyPropertyChanged("IsChecked");
            }
        }

        protected void NotifyPropertyChanged(string strPropertyName)
        {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(strPropertyName));
        }

A have a Collection of SItems (SItemCollection), which fills my ListBox with items, some of which are ticked. This collection is outside the SItem class, it is inside my view model. I also have a set of all items (AvailableSItems) that should be available in the ListBox and a set of items that should be checked (ItemsToBeTicked) at the very beginning. This two sets contain objects of type CachedStr. By using those sets I get my SItemCollection:

public ObservableCollectionEx<SItem> SItemCollection
    {
        get
        {
            ObservableCollectionEx<SItem> strItems = new ObservableCollectionEx<SItem>();
            this.AvailableSItems.ForEach(p =>
            {
                SItem item = new SItem();
                item.Item = p;
                item.IsChecked = false;
                strItems.Add(item);
            });

            strItems.ForEach(p =>
             {
                 if (this.ItemsToBeTicked.Contains(p.Item))
                 {
                     p.IsChecked = true;
                 }
                 else p.IsChecked = false;
             }
             );
            return strItems;
        }
    }

The above-mentioned code works. But I also need a method which will get the final set of all ticked items (after, for example, pressing the button), and that's where I'm stuck. I do get a notifications when I tick or untick something. Please help!