You should be able to get one from Halfords or any decent motor accessories shop. I have bought some for quite a few Fords. They come with adhesive pads on the back - a very simple job provided you've got the original (cracked?) mirror in place ?
We got one from Halfords too, but would like to suggest that if you have some wide sticky backed plastic, place that over the base mirror to mesh it together as much as possible before sticking new over mirror on. We suffered some irritating movement as next town riddled with sleeping policemen.