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

Myeongdong Station Store (134-1 Toegye-ro), both open 10am–10pm. The acrylic mirror S is ₩1,000 with 4.2K+ orders, lighter than glass for your suitcase. Not every store stocks every mirror, so try the other one if needed; they're about 300m apart.