In Myeongdong, go to Daiso Myeongdong Main Store (43 Myeongdong-gil) or

Myeongdong Station Store (134-1 Toegye-ro), both open 10am–10pm. Color names are often loanwords like 그린 (green), so they're easy to read. The mixed-color acrylic yarn in green (50g) is ₩1,000 with 2.7K+ orders. Yarn is light, so it makes a nice souvenir too.