There are a huge number of English job boards on the internet. Most list teaching positions all over the world, but there are a few dedicated to jobs in Korea. Here is a list of some of the most popular:
- Dave's ESL Cafe (by far the most popular, but can be tiresome to read through all the posts)
- Korea Job Finder (allows you to search jobs by location, hours, salary, etc.)
- worknplay.co.kr (especially useful if you're looking for part-time work)
Those are the main job boards in Korea. If you can't find a job between those three, there's a list of more job boards at 1stopkorea.com, though some of the links may be outdated.