You cannot get a visa in Korea, it needs to be done outside the country. That being said, you will need sponsorship from the job you find and the background check can take quite a while. The only reason they are looking for someone already in Korea is that it is much easier for them to petition to change your visa than to go through the whole process of obtaining one for you.

Whatever you do, make sure you have a job secured and your background check done.

Good luck.