Check out Fleabay,you can get some pretty good ones one there. Elsawin are the ones to go for, they are dealer spec. and cover every model. They only cost about £5, but it can be a bit of a lottery getting one that will load properly. It took me 2 attempts, but well worth it.
Short of that have a look on Amazon for Bentlys manuals, thay start at about £40. They are dealer spec manuals and come from the USA, consequently diesels are not covered as they are not very big in America.
Haynes don't do them, as the A6 is not deemed popular enough to bother printing one!!