Have you tried the COSTCO in Suyeong? It's probably only going to be available in massive quantities (like that episode of The Simpsons where those massive containers of cranberry juice break open and cause a tidal wave of cranberry juice down the aisles...ha)
Beware the Cran-tastic tidal wave in COSTCO!
in addition to all of the other places folks have mentioned you should check the specialty food shop in shinseggae (however you spell it). the cooler area has a decent selection of beverages. they carry some santa cruz organics, which are pretty groovy. fail to remember if they have straight cranberry juice off the top of my head. since you live in seomyeon, the vegan buffet place would be worth a look. you can get directions somewhere on the old forums or just go to www.happycow.net and get them there. they carry some stuff. when and if you find what you are looking for exactly, please share the information here
Costco has the best deal on cranberry juice, big jugs of it..
Also, you can get it at most large grocery stores. I see it in Megamart and HomePlus.
No need to pay exorbitant prices at Shinsaegae.