Just came from Korea and stayed there for a couple weeks.
Stussy just housed itself in Seoul so yeah that's around Gangnam/Sinsa-dong
What you'd want to check out though for a more international streetwear brand variety is this place called A-Land. I've found 3 shops mainly in Myeongdong and 1 in Itaewon. It houses a lot of brands many of us would definitely like to sift through. Also around the Myeongdong area you might find a couple of little shops, just browse around.
Another place to note is Abbey Street, also in Myeongdong. The whole floor is full of big streetwear brands like HUF, The Hundreds, Stussy, Nike SBs, Herschel Supply Co, Clae, and the list goes on. I met one of the owners of the stores there - Waylon:Shop. Might as well check them out on FB (/waylonshop) . Also search for IconSupply Co
Dongdaemun also has a couple of big plazas filled with clothing - raw and finished. There are a lot of caps in this area, ones you could probably simply buy and brand.
Then try to checkout Hongdae (Hong-ik Univ). The place is bustlin at night with lots of activity. Also some good clothing finds here. There's also a Kasina in this area so check that out too.
As for Jordans, I asked around and figured Jordans are indeed popular but there's not much supply of it in Seoul. It's more of an Adidas and NB type of area. I managed to find 3 areas though
Itaewon - there's a shop just along the main street where it's stocked with a couple of Js with very limited size availability. Also the Nike store there has Js and Kobes in stock. You might get frustrated cuz I think aside from this one in Itaewon, I haven't seen Js in other Nike stores.
Gangnam - form Sinsa station find Garosu-Gil there's also a shop there I forgot the name something like Nike Hoops
Myeongdong - The Footlocker near Lotte Young houses Js but nothing much to note about.
That's as far as my feet went.
Hope that helps