i know i've seen this somewhere before, but I can't remembver. i would love if it someone could post a linky for me. THX

If anything you should get them for the front. The rear brakes are used less when braking, thats why they are so small. I'm assuming you already upgraded the fronts, but it really isn't necessary to do the rear. Remember its a front brake biased sytem, putting bigger brakes in the rear and not the front would just through off the balance. Unless you like drifting.

ECS tuning has them here.

For the price of upgrading your rears to S4, you could get the front Boxster kit from Pure Motorsport and it would actually make a positive difference.

Front S4 brakes won't fit on a B5A4. They'll only fit on the B6/B7 A4 in front.

Rear S4 brakes are a direct fit on the B5A4 Quattro only. But you'll need rotors, carriers, and calipers, which makes it a pretty expensive upgrade.