The bulb is about $225 from a VW dealer. The last time I checked (about 8 months ago) it was not available aftermarket.
If you know how to remove the headlight assembly, it is not difficult to change the bulb yourself.
There is a tool in the on-board kit which is used to unlatch the over-center lock holding the light assembly in. (All it is, is a 6mm socket.) Turn the over-center lock GENTLY counter-clock-wise until it pops. It will be loose and rotate freely about 3/4 turn.
Then reach in behind the assembly and push it forward. It will slide about 3 inches forward where it reaches a safety catch, which you can depress with your finger or a screwdriver; then it will slide out. The wiring automatically dis-connects.
Open the rectangular cover (with the lightning bolt) on the back of the assembly and R + R the bulb. DO NOT touch the glass of the new bulb; skin oil will cause early failure.
When you slide the assembly back into it's tracks the wiring connecter will automatically line up. Once it is fully seated, push in on the front glass of the assembly, so that it will be easier to lock it, and rotate the over-center lock GENTLY clockwise until it pops into place.
CAUTION: The over-center lock will break if you apply too much torque. Go easy and listen for the pop. The right one unlocks counter-clock-wise and locks clock-wise. The left one is opposite: it unlocks clock-wise and locks counter-clock-wise.
A search of the site should get you more info and possibly some photos.
Look in www.files.mytouareg.info