Hannam has the best foreign market in Seoul.  Go to Hangangjin Station (항강진역) and leave through exit 2.  Head straight under the highway.  Cross the footbridge and follow the street right.  Look for a Volvo building on your left.  Head downstairs and you're there!  Be sure to check all three markets in this building for what you want.  There is one large one, one that is like a small deli, and one that looks more like a gas station market than anything else (but they have some great ingredients that you can't find anywhere else.  Almond extract for instance.).