Hi,
The best my be to store normals on points instead of cells. You can get those with the vtkPolyDataNormals filter. Then you can get the point normals with polyData.getPointData().getNormals().getData()
Should you want normals on cells instead, the best would be to improve vtkPolyDataNormals (PR is welcomed) to conditionally compute the normals on the cells (not just the normals of the points as it is right now).
Then you would then be able to get the normals from polyData.getCellData().getNormals().getData().
Hth, Julien.