The W6 craftsmanship engineering and HandelsgesellschaftmbH exploited its market position to the detriment of their customers who have purchased such a sewing machine from. Too bad, but it incites creativity to matching coil at other sewing machines (accessories) to find suppliers.
Addendum:
Spare found! The 11.5 mm Plexiglas coils of Brother fit perfectly and can be obtained from each Nähmaschinenfachändler.