Flight
Portland, ME will be the closest city for you to fly into, however, it may be a little more expensive. If you would prefer to have a shorter drive to your hotel, you may want to choose this option.
Flight
If you would like to find cheaper flights, Boston, MA will usually be more affordable to fly into. You will have to pick up your rental car from there and drive a couple hours to get closer to the wedding site, however.
House Or Rental
There are tons of Airbnb locations around southern Maine. It would make the entire trip more affordable if you could book one of these and split the cost among multiple people.