Audi do not sell the bushings for the Audi A8 front anti roll (sway) bar separately. They will sell you an entire front antiroll bar with premouded bushings.
If you dont want to pay for the several hundred $$ part, print your own. Note: These need to be very tight! Remove the original rubber, which is no doubt broken in parts. Scrap bits off the bar and then sound down with coarse sandpaper. You may need to warm the parts and lubricate with WATER ONLY. You dont want them to slide around afterwards. You may also need to use a crow bar or similar to "snap" them around the bar and then slide into place.
Nylon filament will work best here. You can change the density to adjust the "firmness", recomment 70% and above.