Question

Where about is Razor Fang on Mirage Island?

Answer 1

In every Mirage Island, you must use the Dowsing Machine in order to obtain it. In the West Dewford Mirage Island it is behind a tree. Good luck finding it!

Hope I helped. :)
Source: http://serebii.net/pokearth/hoenn/mirageisland.shtml

Answer 2

Each day there's a hidden item on each Mirage Spot, and you should use the Dowsing Machine to find the exact location once you land in the spot.

As for the exact islands where Razor Fangs can spawn, check the map image below and see if your current mirage spot matches any of the dots in this map.

(Source: Guide Strats)