Very difficult to find. I had the same issue:http://sciroccoregister.co.uk/carforum/viewtopic.php?f=6&t=19260&p=133826#p133826
For a lot of original ones, the plastic housing seems to have perished. If you're lucky enough to find one on eBay/Amazon, get them to check the plastic housing is still solid but you'll probably be £50-£70. An alternative is to use a Golf Cabrio mirror, which has the same "metal plate bonded to windscreen" mechanism, even though the mirror itself is slightly different. They're maybe slightly easier to source. See this post http://sciroccoregister.co.uk/carforum/viewtopic.php?t=15740
In the meantime, if you feel safer driving with a mirror, it may be worth get a cheap £5 stick-on mirror as it might take a while to find a proper one!
Perhaps ask nicely in the "Parts Wanted" section.