I had the same cog wear out on my old D3, I couldn't find parts and was tempted to buy a throttle body off a golf to see if the cog was the same (much cheaper than an A8 one, but a different shape!) but in the end bought a pair of throttle bodies of a German breaker that only have 30k kms on them and just swapped the full units.....
Still have the "spare" good one in the garage (and likely the broken one also)