I experienced same situation
פורסם על ידי Ai Le ב- דרום קוריאה פורום
Incheon airport has a few counters you can pick one up at however I strongly recommend you reserve one online. Widemobile all the way. Cheapest unlimited data unit available. However they rent theirs out super fast so there is a chance they don't have any available. There are other options like KT and Olleh but they are a bit more expensive for unlimited data.