Coincidentally I fitted 4 new caps yesterday to my freshly refurbished wheels.
Bought them from a company based in China called Rhino Tuning and cost $31 including shipping to the UK. They accept PayPal so no need to worry about credit card details.
They arrived amazingly quickly bearing in mind they came half way round the world.
They're not perfect but look brilliant once on the car.
There's no way I'm paying Porsche prices for 4 pieces of plastic!