They rarely come up for sale and when they do they're snapped up straight away.. They also cost a fortune from the dealers.. There was a couple of secondhand ones on ebay a couple of weeks ago, but one had a slight crack, and the other was a tad overpriced for the condition.. It was a bit grubby and he still wanted a bit for it..
Just incase you want the part numbers:
8L9103928B(01C) Expansion tank cover 8N0103946A Retainer 8L9103927A Battery cover 8L9103912A Holder for the battery cover 8N0103531 Screw (3 of these)