For starters, I have no affiliation here other than being a customer of this ebay seller. Confirmed 12V 21W Nokya Hyper Yellow 1156 (7506) bulbs. You'll see in the pictures posted on ebay that the bulb is stamped "21W."
This is representative of what they look like installed in the bi-xenon headlamps on a 2007 A3 S-Line.
$16.95 for a pair of bulbs with free USPS shipping is about the cheapest you're going to find these bulbs. bit.ly link to ebay listing: http://bit.ly/HyperYellow1156