Online parts catalogues
 

A few catalogues that may help source that elusive part or give you the part number you need. You know any more let me know and I'll add them.

VAGCAT
Nearly all the part numbers from nearly all VAG vehicles. You need to register, but its free


Meyle
Very comprehensive parts catalogue with multiple search methods. Very useful for cross referencing Audi part numbers. e.g. A single Meyle part number will be listed against all the VAG part numbers it replaces. With this you can ascertain if the Tie Rod end for a Passat will fit your A8.
I found this very useful when researching poly bushes as I found that the majority of parts are interchangeable with A4s of various vintages and therefore identified poly bushes that would fit the A8, although may not be recommended.

Bosch
For all those service parts you may need. The ubiquitous Bosch parts catalogue for the D2s and D3s

Lucas
For all those electrical Mods and repairs.

Mike