Winter floor mats are available for the A8. The mats shown below are "euro style". They do not have any markings on them, i.e. Audi or A8, but they are very nice.
They are approximately $100 for a set of four from your local Audi dealer. Always ask for a discount, they will usually give you ten percent off. These hold a lot of snow/water and attach to the existing front mounts. Rears have a velcro type surface to stay put. They are highly recommended!
Part numbers for the mats are as follows:
Front 4D1 061 501B 041
Rear 4D0 061 511 041
If you have an A8L, they will also fit nicely.
A8 Winter Driver's Euro Mat
A8 Winter Passenger's Euro Mat
A8 Winter Back Seat Euro Mat
Also shown are the regular floor mats, they are a deep ribbed channel design with model number logo. I do not believe these are availble for the A8, only for the A4, S4 and A6.