Where have you tried so far?
You could try eBay, or the old fashioned way and Google local breakers yards that might have a VAG group vehicle with the same engine.
Definitely don't rule out aftermarket upgraded items from pioneer, JVC etc.
Have a look at this regarding B&o speakers:
https://www.audiownersclub.com/forums/topic/25250-b-o-is-it-genuine-or-just-a-rip-off/
Nice one, sounds like the A6 is in good hands. Whilst the front is off for the cambelt and water pump it might be worth replacing the lower pulley as mine nearly fell off! And the thermostat whilst he has easy access.