I bought a set of two steel pins similar to the below on ebay
https://www.ebay.co.uk/itm/231466008055?
For anyone 'downgrading' to steels due to expensive ceramics you can use 2-piece 380mm discs from the Porsche Cayenne Turbo S (this car uses the same Brembo 19z calipers as an option) and swap over the bells for correct hub fitment. Or any 380mm two piece (Nissan R35 GTR etc... although 35/34mm) with a custom bell. Then use pads that meet these part numbers below:
Pads:
AUDI: 7L5698151
PORSCHE: 95535193950
PORSCHE: 95535193951
PORSCHE: 95535193952
Wear sensor cable:
AUDI: 4E0615437A
PORSCHE: 95561236551
Porsche 380x38 2-piece discs:
PORSCHE: 95535140161 Left
PORSCHE: 95535140261 Right
As for the S8 rears to steel, you can get a set of W12 rear caliper carriers and go OEM route for discs / pads.
With the above you use the existing calipers. Nice and easy. Still expensive but no where near new ceramic price.