Можно использовать CollectionViewSource в качестве ItemsSource. Например так:

<Window ...
  xmlns:ComponentModel="clr-namespace:System.ComponentModel;assembly=WindowsBase">
  <Window.Resources>
    <CollectionViewSource x:Key="viewSource" Source="{Binding SomeSource}">
      <CollectionViewSource.SortDescriptions>
        <ComponentModel:SortDescription Direction="Ascending" />
      </CollectionViewSource.SortDescriptions>
    </CollectionViewSource>
  </Window.Resources>
  ...
  <ListBox ItemsSource="{Binding Source={StaticResource viewSource}}" />
</Window>