I can't find any OEM rubber floor mats for an A6 4.2, can anyone help?

Try ebay

They usually have everything

Try Clair Parts (http://www.clairparts.com/) -- that's where I got mine.

or ECSTuning.com. By the time you tack on shipping though it's not too much less than the stealership.

I ordered mine for my '01 A6 2.8 direclty from audiusa.com. After shipping they were about $114, but they are rock solid, and I feel worth it.