Seattle–Tacoma International Airport (SEA)
Th Seattle Airport is about 15 miles south of Downtown Seattle, where Bumbershoot takes place, but thankfully they have excellent public transportation to the city. For just $2.50 you can take the train, which departs regularly throughout the day (every 20 minutes). You can hop on the Link Light Rail over by the baggage claim area of the airport. It travels to Downtown with several stops within the city, so there should be a stop near wherever you are staying.
$2.50 each way on the Link Light Rail to / from the airport.
Generally the cheapest time to book flights for Bumbershoot will be about six weeks before Bumbershoot. This means you should be focused on booking your flight in July 20, 2018. This may seem late to you planners out there, but statistically it's the cheapest time to book flights. That said, if this is too late for you book it earlier. One final note is that plane tickets for Bumbershoot tend to skyrocket about two weeks out, so certainly don't wait that long to book a Bumbershoot flight.