I would hope that the outer diameter is a standard size, but those notches make them specialised low profile
Along with knowing they are a complete ar$e to fit I am very sure you can't source them from VW - they would just replace the whole caliper
I would try contacting a specialist in braking such as Big Redd or Brakes international.
If you get a replacement, check they have seated properly before rebuilding the rest of the caliper... don't ask how I know
