Use Expression Blend, edit a copy of the current template for the Expander, go to XAML for the template, rename "ExpanderLeftHeaderStyle" to "ExpanderRightHeaderStyle" and "ExpanderRightHeaderStyle" to "ExpanderLeftHeaderStyle".

I prefer to use a DockedExpander class I wrote a while back (the code is included below). This class automatically sets itself up for whatever side of a DockPanel it is docked on.

For example, in:

<DockPanel>
  <edf:DockedExpander DockPanel.Dock="Left">
    <ListBox ...
  </edf:DockedExpander>

  <Grid ...

</DockPanel>

The expander will open from the left, with the button facing the right way. But changing it to:

  <edf:DockedExpander DockPanel.Dock="Right">

will automatically adjust the rest of the expander to match. Same with "Top" and "Bottom" docking.

I implemented DockedExpander because the thought of copying several hundred lines of WPF s internal code into my project was abhorrent to me. Also, my DockedExpander control automatically adapts to new theme styles because it reads WPF s internal styles.

Here is the code for the DockedExpander class:

public class DockedExpander : Expander
{
  static DockedExpander()
  {
    _directions = new Dictionary<Dock, DirectionData>();
    _directions[Dock.Left]   = new DirectionData { Reverse = Dock.Right,  ExpandDirection = ExpandDirection.Right };
    _directions[Dock.Right]  = new DirectionData { Reverse = Dock.Left,   ExpandDirection = ExpandDirection.Left  };
    _directions[Dock.Top]    = new DirectionData { Reverse = Dock.Bottom, ExpandDirection = ExpandDirection.Down  };
    _directions[Dock.Bottom] = new DirectionData { Reverse = Dock.Top,    ExpandDirection = ExpandDirection.Up    };

    DockPanel.DockProperty.OverrideMetadata(typeof(DockedExpander), new FrameworkPropertyMetadata
    {
      PropertyChangedCallback = (obj, e) => ((DockedExpander)obj).UpdateExpandDirection()
    });

    ExpandDirectionProperty.OverrideMetadata(typeof(DockedExpander), new FrameworkPropertyMetadata
    {
      PropertyChangedCallback = (obj, e) => { throw new ArgumentException("Cannot set ExpandDirection because DockedExpander always computes its ExpandDirection from the DockPanel.Dock property"); }
    });
  }

  public override void OnApplyTemplate()
  {
    base.OnApplyTemplate();
    UpdateExpandDirection();
  }

  private void UpdateExpandDirection()
  {
    // Can t use GetTemplateChild because non-PART_ names are not guaranteed to stay the same
    var dockPanel = FindTwoElementDockPanelUnder(this);
    var headerSite = dockPanel.Children[0];
    var expandSite = dockPanel.Children[1];

    // Compute the docking
    Dock myDock = DockPanel.GetDock(this);
    DirectionData myDockData = _directions[myDock];

    DockPanel.SetDock(headerSite, myDockData.Reverse);
    DockPanel.SetDock(expandSite, myDock);
    headerSite.SetValue(FrameworkElement.StyleProperty, myDockData.HeaderSiteStyle);
  }

  private static Dictionary<Dock, DirectionData> _directions;
  private class DirectionData
  {
    public Dock Reverse;
    public ExpandDirection ExpandDirection;
    public Style HeaderSiteStyle
    {
      get
      {
        if(_headerSiteStyle==null)
        {
          var expander = new Expander { ExpandDirection = this.ExpandDirection };
          expander.BeginInit();
          expander.EndInit();
          expander.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity));
          var headerSite = FindTwoElementDockPanelUnder(expander).Children[0];
          _headerSiteStyle = ((FrameworkElement)headerSite).Style;
        }
        return _headerSiteStyle;
      }
    }
    private Style _headerSiteStyle;
  }

  private static DockPanel FindTwoElementDockPanelUnder(DependencyObject visual)
  {
    while(true)
      switch(VisualTreeHelper.GetChildrenCount(visual))
      {
        case 1: visual = VisualTreeHelper.GetChild(visual, 0); continue;
        case 2: return visual as DockPanel;
        default: return null;
      }
  }
}

As usual, you need a namespace declaration (xmlns) in your XAML to be able to use a custom control.