There are two ways to get to Phu Quoc Island. By ferry or via Phu Quoc International Airport.
If you're planning to fly, I recommend Kiwi.com. It works like Skyscanner, and often you see cheaper flights from Kiwi in Skyscanner's results.
With Kiwi.com, you can also book your accommodation. I find having everything in one place super convenient when it comes to recalling confirmations, etc. You can also set up price alerts to take advantage of any discounts before you book your flight.
From Outside of Vietnam
If you are coming to Phu Quoc from outside of Vietnam, it's best to arrive at Ho Chi Minh City and then catch a connecting flight to the island.
There are several internation flights from other parts of Asia, Europe, and Australia. Check Kiwi.com whether or not you can get direct flights from your country.
Depending on where you are in Vietnam, there are several ways to get to Phu Quoc.
If you are anywhere north of Ho Chi Minh, then I highly recommend flying. You might save a few dollars on a night bus, but you'll spend more than a day traveling.
You can get direct flights to Phu Quoc from Hanoi and Da Nang. Most other cities will include a stop in Ho Chi Minh City.
Ho Chi Minh City to Phu Quoc
From Ho Chi Minh City, you can either fly or catch a bus to Phu Quoc. A bus will take around 12 and a half hours and cost between USD$30 to $40.
There are several direct flights from Ho Chi Minh City to Phu Quoc daily and start from around USD$60 for a one-way ticket.
Siem Reap to Phu Quoc
The quickest and most affordable way from Siem Reap to Phu Quoc is to fly.
Unfortunately, there are no direct flights, so you'll have to fly Siem Reap > Ho Chi Minh > Phu Quoc.
Koh Rong to Phu Quoc
You have a couple of options when traveling from Koh Rong to Phu Quoc.
You can travel by bus, which takes around 8 hours of travel time and costs about USD$40 to $50. The bus takes you over the border just north of Hà Tiên (Ha Tien).
Depending on which bus service you book, your journey might end at Ha Tien, in which case you need to catch the ferry over to Phu Quoc. A Grab from the ferry port to central Phu Quoc (Duong Dong) will cost about USD$7 - USD$12.
The quickest way from Koh Rong to Phu Quoc is to fly. You will need to catch a bus or taxi to Sihanoukville International Airport. From the airport, you can fly direct to Phu Quoc International Airport, which takes about 50 minutes or so.
A direct flight from Sihanoukville to Phu Quoc on Vietnam Airlines costs around USD$80, including 20kg of checked baggage.
Krong Kep/Krong Kampot to Phu Quoc
From Krong Kep or Krong Kampot to Phu Quoc, it is best to take a bus. You can book a bus at one of many local travel agents in Kep or Kampot.
Expect to pay anywhere from USD$20 to $35 from Kep and USD$50 to $70 from Kampot.