If you have your owner's manual packet, look in there for a card. Also, look in the owner's manual for the radio. My previous owner was nice enough to write it in the section that dealt with the code.
If it's not in either of those places, contact your dealer. I have seen an online website that will retrieve your code for $29, but I have not verified that that works. Apart from that possibility, it's pretty much dealer or nothing. Can't even get it with Vag-Com.