Cheap Flight Between Seoul and Tokyo

I have a few questions about flights between Seoul and Tokyo:
1. I would like to spend a week in Seoul. I am planning to fly one way and take a ferry the other way. Is it cheaper to fly from Seoul to Tokyo?
2. Where can I buy discount tickets? Some airlines do not allow to purchase tickets over the Internet.

Thank you.

Flights between Japan and Korea are extremely numerous and can be very cheap. There are flights to Seoul not only from the major airports, but several minor airports across Japan.

Here is a good agent with offices in several Japanese cities:
http://www.no1-travel.com/

And one of the ferry companies' English website:
http://www.jrbeetle.co.jp/english/