Maybe it is a sort of snap in plate with a self tapper style threaded hole in the centre that you need.
I've forgotten which part these plates are fitted to, so until I find that out I would never manage to find a part number, if you know exactly which part they are fitted to then maybe get your local VW/SEAT/Skoda dealership to order some in for you and report back with the part number so that others know it.
Edit again!:- I think that it is better to create this sort of thread in the main area for this type of
Ibiza as it will always get better coverage, and so quicker/better/more replies - well that is my way of thinking anyway.