Remove one and take it to a specialist fastening shop, ie, in Bournemouth there is an allfix distributors who do almost every fastening, bolt, screw, washer etc known to mankind. A local place to you like this one may be able to help. I was even able to buy 3 piece wheel tie bolts for my Cobra from a place like this.
If not, main dealer like VW, Audi or Seat or maybe even Skoda.