Euro Car Parts (UK)
GSF car parts (also UK)
www.gsfcarparts.com
ECS Tuning (USA?)
https://www.ecstuning.com/
Your local Audi dealer? surely they can order parts in from Germany if required?
The UK Audi dealers order parts from Germany when they don't have them in the UK.
I remember some guys on the forum fitted polybushes. Not sure if it made the ride harsher than stock rubber bushes.
http://www.polybush.co.uk/