The easiest for you is to contact our friend Mr Ritty 012707719. You can visit him in his office by the Riverfront. He can help you arrange shared taxis, that should be about $10. The trip should normally take 2-3 hours.
You can also contact us if you need a private taxi, from $80.
The taxi will take you to Poy Jopon. From there you take one of the small speedboats to the Island. When you share the boat with others the normal price is 5000 Riel or $5 for your own boat.