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.
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.
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.