When we were in Seoul, we made a trip to the Cheonggyecheon electronics market, a maze-like network of small streets and shops selling LEDs, electronics and other supplies such as outdoors carpeting, fire extinguishers, aluminum profiles, etc. That day, I was looking for a bit of EL wire (electro-luminscence wire), and we went around with the simple goal of to finding out the estimated price of LEDs and EL wires.
The place is like Sim Lim Tower but spread out over many little small winding streets. In the end I discovered that EL wire is not common in Seoul, but certainly many types of LED strips and boards can be bought and customized here. We received a quote of around 60000 won for a 5m LED strip and an additional 30000 for the inverter. If you can bargain in korean (which we fail totally at doing) you might get a better rate than this.
But if one is in Seoul and looking for parts, this is probably the best market I've seen there. Looking online for more information, I also found that someone had made a great map of the market here on dangerousprototypes.