Transportation from Helsinki airport: https://www.hsl.fi/en/timetables-and-routes/terminals/transport-links-helsinki-airport
Easiest way is to take the I-train to Tikkurila railway station. You can then continue to Seinäjoki by train.
Tampere Airport (Ryanair)
There is a airport bus operating between Tampere airport and the city center (railway station). You can take a bus to Tampere railway station, and continue you journey to Seinäjoki by train. http://joukkoliikenne.tampere.fi/en/info/for-tourists/airport-connections.html
From Vaasa Airport you can easily reach Vaasa railway station by taxi and continue from there to Seinäjoki by train. You can also take a taxi directly from Vaasa Airport to Seinäjoki; the trip takes around 1 hour and costs around 24 – 80 € depending on if you use private taxi or the airport taxi. Airport taxi line is served by Pahkakankaan Liikenne and can be booked in advance at the address: email@example.com
Helsinki – Seinäjoki (approximate duration 2 hours 45 minutes)
Tampere – Seinäjoki (approximate 1h 10min)
Vaasa – Seinäjoki (approximate 45 minutes)
Please check the schedules from VR’s webpage: https://www.vr.fi/cs/vr/en/frontpage
Seinäjoki railway station is located in the center of the city, so many accommodation options are within walking distance. Seinäjoki taxi center is located at the railway station (in front of the station entrance). Telephone number for taxi centre is +358 100 84 999.
More information ABOUT Seinäjoki and the region can be found here: