I don't know if they stock it at the moment but I got some great double sided sticky tape from aldi
(It's as good as the Harris number plate tape, in Halfords, at a quarter of the price)
Do you mean just the mirror or the plastic backing plate as well that the mirror is fitted into?
You can either glue a replacement onto the backing plate or as I did remove the backing plate, heat it up so its flexible then fit another mirror into it. Its tricky but its as factory. I can't stand just sticking a new mirror on top of broken ones. But I'm ocd.
Yes it's the plastic backing plate that's clipped on. You have to lift up one side and then slide it to the side slightly. Trouble is you can't bend the clip enough to slide it with the glass fitted. This isn't a problem if there's no glass in it though.