In korea it seems to be Seoul (the captial) and Busan (another big city) (i see a lot of korean cube forum posts from people from Busan)
And of course Changwon, only because cubenjoy is there and ships from there.
But korea is such a tiny country anyway, just a bit bigger than Kentucky area wise.
It does have about 50 million people though.
NOTE: All mentions of Korea, I mean South Korea. I highly doubt cubes even exist in North Korea.