The nearest and main airport is Helsinki-Vantaa airport.
The bus leaves from the airport, outside terminal 2.
Don´t be surprised if you have to change the bus in Lahti (the city in about half the way).
The journey takes about 3 hours.

Here is the site where you can see the timetables:
Matkahuolto - Bustickets
and Sanna is also happy to help you with these questions!

Wellcome to Finland! = Tervetuloa Suomeen!