I got one for my B6 A4 for about £400 from Ebay Germany, widen your search to international and translate the language via bablefish so you can communicate with the seller and read about the Nav+ unit.
You also need to find out exactly which model fits yours, this is very important or you will need to do a lot of messy modifying.
I dont think Germany accepts paypal so you will need to pay via bank transfer which cost me about £20.
If the seller sells various items as well as nav+ units, get a GPS receiver to suit the nav+ and the cable adaptor harnesses too.
Last thing, you will also need a CD changer if you havent already got one.
HTH
Mike.