I cannot see your image,
Do yo mean part number 12 or 14 in the diagram?
Thispage is taken from the Panels, Underbody section of the online parts catalogue.
http://www.catcar.info/opel/?lang=en&l= ... xpa2V5PT01
It is a bit of pot luck.
Most parts are no longer available
But on the odd occasion there might be a panel lay in the corner of a dealership collecting dust.
If there is, try giving GPS (genuine parts search) a call with the part number.
https://www.genuinepartsearch.co.uk
GPS managed to get me a panel (14 in the diagram) from a dealership in Liverpool in 2016.
Otherwise, try a panel supplier, such as Carz2
https://www.carz2.co.uk/vauxhallopel-ve ... 6608-c.asp
I would suspect however that any "old" stock is probably long gone, so more than likely, you will need to fabricate this yourself.