Anthea,
The Isofix bar part number for your car is 8D0813545 and the bolts are 8D0837453A and are described as Hex Socket Head Bolt (combi) M8x27. If audi say they are obsolete(which I very much doubt) get part no. 8L0837453 which is for the A4 and is described as Hex Socket Head Collared Bolt M8x27. I'm pretty sure that the A4 bolts will do the job.
The plastic guides for your car according to ETKA are part no. 8L0019235.
I wouldn't bother with the plastic guides to be honest, they scrunch up the seat and leave gaps on either side. I won't be using my plastic guides at all.