hi , A3 interloper here, but wanting a B6,so doing a bit of lurking
I'm just having the same issue on mine (1.9tdi),
first thing is don't run the AC any more, it may be the clutch pulley, or it may be internal, if its internal then whenever you run the comp it may be distributing bits of metal around the ac system, which means lots more work and/or parts to replace.
there are two styles of compressor on my car, one you can get the clutch assembly separately, the other you cant, of course mine is the non replaceable type. I did find a new compressor for 140 quid, which is not bad, also needed new condenser/dryer because that are full of aluminum paste
maybe if its the same style that's why you cant find a pulley/clutch for it
edit
searching for air con clutch brings up a lot of results like this
http://www.ebay.co.uk/itm/Air-condi...hash=item3f6b181ee8:m:mBGIH36xTU7INFhEvezKR1Q
so it looks like it is a serviceable item,