ECS calls them "Rear Lower Control Arm Bushing - Outer" Scroll down about 2/3 of the way here:

OEM / Performance Parts for Audi, BMW, Mercedes Benz, Mini, Porsche & Volkswagen - ECS Tuning

 

I have just finished the same right rear wheel bearing replacement on my 1996 Audi A4. The bearing that holds the bolt on the bottom of your knuckle is built with a metal shaft inside that rusts right to the bolt. I used heat and a hand sledge to remove it. The bearings are called Control Arm Bushings - Rear Lower and can be found at

96 1996 Audi A4 Quattro Control Arm Bushing - Suspension - Beck Arnley, Corteco, Febi, First Equipment Quality, JLB, Meyle, Sachs, Vaico, Front, Front Lower Inner, Front Lower Inner Front, Front Lower Outer, Front Lower Rear, Front Upper, Rear Lower,

You will need to buy 2 of these to fit on both sides of the knuckle. Audi dealerships will have their special bolt with the offset washer mount.

 

Happened to one of mine also. When I would spin the bolt, the bushings would spin also. Finally used an angle grinder and a cut off wheel like someone else mentioned. Got 2 bushings at napa.

 

Got the bushings today, bolt gets here tomorrow, had to go to audi to get it, 30 bucks for the damn bolt

 

Yeah they kill you when it's a special bolt. That's the eccentric bolt for camber adjustment.