If it is a similar plastic coolant flange to the one in my wife's Golf Mk5 TDI there is no gasket, it uses a rubber o-ring instead.

You probably need to go see the parts department of your nearest VW dealer to get a "quote" on the plastic flange, they should be able to give you the VW part number (in the format XXX XXX XXXY, Xs are numbers, Y is letter) and you should then be able to search ebay using that part number.