This is the closest International Aiport. Rent a car and drive to the ferry in Anacortes. Plan on 2 hours to take into account Seattle traffic.
Note: See details on the site for the option of flying into Seattle and then Boeing field to take a small plane right into Eastsound, on Orcas. There is also a car rental on Orcas and they bring the car right to the Eastsound airport for you.
Paine Field in Everett, WA
Instead of Seattle, you might consider flying into Paine Field, a small airport in Everett, WA. You can rent a car from there and drive to the ferry landing in Anacortes. Plan travel time of about 90 minutes.