Some good news for you my friends.   Oatmeal and cilantro are here!  Get yourself over to the new foreign  food market, right by Cheonan Station.  And while you're at it, stop by  the most delicious Indian restaurant and support other foreigners in  town.
Directions:
From the Cheonan Station courtyard side  (Dongbu-Exit 1), walk down that big road towards the bus terminal for  about 1 minute.  The "World Food" store will be on your right hand side.   You can't miss it.
Finds:
Cilantro, a big bag for 2000  won.  Those big jug things of oatmeal for 15 000.  A nice selection of  random spices.  Dried beans of all varieties.  Import beers.  Thai  noodles, rice papers, etc.  Check it out...I promise you'll come away  with something.