So I just visited Korea a few weeks ago and while I was there I went to Nike Seoul, Nike Gangnam, Nike Hongdae to customize some shirts and caps. Each location has some patches and prints that are unique to the location, and the amount of each product that you can purchase also depends on location (Gangnam only allowed 2 hats per person while Seoul allowed 3). Generally all 3 locations for us had staff that were a bit limited in English but not to the point where you can’t get your point across. The patches and prints range from 3,000-10,000 won depending on the size of the print. The shirts, long sleeve tees, hoodies and caps were available at each location. The quality of the tees are kinda lacking as they are a bit thin IMO. The long sleeve and hoodies were fine. My biggest advice would be to look at the examples that are hung up and peek at the stickers and prints available on the tables prior to starting your custom items! You only get 30 minutes which is not a lot of time at all when trying to design. If your ambitious and are trying to make multiple different items time will definitely be a factor.
Also stopped by Adidas Hongdae and made some stuff. There selection is definitely smaller but the quality of the tees were much better than Nike. There was no line and we went to make some adidas shirts while waiting for Nike stuff to get made. New Balance also had some interesting options in Myeongdong but at that point I was all customized out.
Nike Seoul (in Myeongdong): Had the most helpful staff and large selection of prints to choose from. If you want to be the first 10 or so in line you have to be there around 8 am and they will pass out times to come in at 9:30-10am. This was the only location that displayed what clothing items were available prior to signing up for a time slot.
Nike Hongdae: Most unhelpful staff out of the 3, more so like they all hated their lives but were kind of nicer once we started speaking Korean? They had almost as many prints and patches to choose from compared to the Seoul location and definitely more than Gangnam. This location opens at 11 while all others open at 10:30 but has little to no line at opening.
Nike Gangnam: Staff was pretty helpful. We lined up about 50 minutes before opening and were 6th in line. May have the least amount of time slots, I believe it was 2 time slots per 30 minutes. The Seoul location seemed to have more tables.
Hope this helps. Happy Customizing