Flight
If you're coming from out of town, fly into San Francisco or Oakland. You'll probably want to rent a car for the 75-90 minute drive from the airport to Sebastopol. The Sonoma County Airport (STS) is just 20 minutes away, but has limited flights from Alaska, American, and Avelo Airlines.
Travel Note
If you aren't renting a car, Groome Transportation operates shuttles between SFO/OAK and nearby Rohnert Park or Santa Rosa, but you'll still need to get a Lyft/Uber to Sebastopol for the wedding. The shuttle fare is $52/person, one-way.
Hotel
One option for lodging is to book one of the many hotels in Santa Rosa close to where Highway 101 meets Highway 12. Or in Rohnert Park close to Highway 101 and Highway 116.
House Or Rental
You can also try searching Airbnb for rentals close to Sebastopol, CA.