📝 Note for collectors
Don't see a point of adding them here individually, look them up on Naver Maps - most of them are in a city center on the north from the river, and just two to the south - one by Yeouido Hangang Park, another one in Gangnam.
📍 얼씨구마당
Souvenir shop on the northern end of the Insadong-gil street, has a good selection of pins about Korea and some about Seoul. See photos: [1] and [2].
📍 Dongdaemun Design Plaza, Design Store
In Dongdaemun Design Plaza complex there is a gift shop which sells a big variety of various designer pins with the theme of Seoul, as well as DDP itself. Also have some pins which are a big more generic. Photos: [1], [2], [3].
📍 Korea Traditional Souvenir Center
Souvenir shop which has a limited choice of pins with a theme of South Korea, but nothing about Seoul. Pin photos from there: [1].
📍 Souvenir shop on Insadong-gil Street
Souvenir shop which seemingly has no point on Google Maps (but it's somewhere near the map location), which sells a big variety of pins in a style of flags combination, like flag of South Korea with a flag of another country - plenty of flag combinations to choose from. But they have no pins about Korea or Seoul.