Depending on where you live in Korea, you won't have a problem getting road tubes and tires but if you live in a small city you may have to mail order them. However getting an MTB here is a great idea because there are tons of trails and you will have a much easier time finding riding buddies since its really the only type of cycling that is popular here. Before I come back for my second year, I'm going to pick up a cyclocross bike so I can have the best of both worlds.
P.S. If you have any questions about living here, PM me.