If it's just the glass that's gone, ie you have a backing panel, you can just buy a replacement mirror glass from halfords as a quick solution, they just stick on to the old bit, but if the backing has gone, you'll have nothing to stick to, so you'd have to get replacement from a dealer or get one from a scrapyard.
Someone actually snapped the wing mirror off my dads car twice in a month, each time we got another from a scrappy.