When you say "unit", do you mean the entire internal parts inside the
mirror "shell", or do you mean an entire mirror (including the colour
coded shell) ?

Yes, I mean everything that hangs off the door. Generally spares are
available as mirror glass, or the complete thing. Obviously mirror
glass is quite a bit cheaper, but unless it's a straight in fit, I
wouldn't bother.

Colour coding on mirrors is a PITA. My VW van would have had them
originally, had a pair in black when I bought it - and I've since had
to replace one - the whole unit replacement consists of 2 electrical
connections and 4 screws.

They're hardly mega-money on ebay:
http://shop.ebay.co.uk/?_from=R40&_t...All-Categories