Here is a piece of code that will retrieve all GridItem objects of a property grid:

public static GridItemCollection GetAllGridEntries(this PropertyGrid grid)
{
    if (grid == null)
        throw new ArgumentNullException("grid");

    var field = grid.GetType().GetField("gridView", BindingFlags.NonPublic | BindingFlags.Instance);
    if (field == null)
    {
        field = grid.GetType().GetField("_gridView", BindingFlags.NonPublic | BindingFlags.Instance);
        if (field == null)
            return null;
    }

    var view = field.GetValue(grid);
    if (view == null)
        return null;

    try
    {
        return (GridItemCollection)view.GetType().InvokeMember("GetAllGridEntries", BindingFlags.InvokeMethod | BindingFlags.NonPublic | BindingFlags.Instance, null, view, null);
    }
    catch
    {
        return null;
    }
}

Of course, since this is using an undocumented private field of the Property Grid, is not guaranteed to work in the future :-)

Once you have all the GridItems, you can filter them using the GridItem.GridItemType property.