Because of the focus in Korea on English as a Second Language in the school system, it’s not terribly difficult to find books in English. It can be difficult to figure out how to find books in English as well as education materials. There are three places that you can use to jumpstart your search: Aladdin, a used bookstore; Kyobo Bookstore, a bookstore that sells brand new materials; and Alpha Store, a supply store that will have a variety of school supplies.
Aladdin is a used book store with several locations all across Seoul. It has a surprisingly large section for English books that encompasses an entire wall and more. You can find their website here (https://www.aladin.co.kr/home/welcome.aspx), where you can check ahead to see what books are available where. Aladdin holds children and adult books, test preparation books, and so on. However, since it is a used bookstore, it’s always worth a visit to see what has newly come into the store.
Kyobo is by far one of the more popularly known bookstores. Kyobo bookstores are very large and have tables and bookcases filled with books and other toys. However, most of those books are going to be in Korean. There is an extensive section filled with English books, test preparation books like that of the GRE and the SAT, and learning Korean as a foreign language books. You can check out the website here (http://www.kyobobook.co.kr/indexEng.laf?mallGb=ENG&orderClick=c1b), but if you go to the English section of a store in person, the staff there can help you with what you are looking for. If there’s a book you’d like, but it is only available online, they can order it for you and have it sent to your home or keep it at the bookstore for pick up.
Alpha Store doesn’t carry books, but it has all kinds of educational supplies that you could be looking for, from stickers to envelopes to games to play with your kids. While Alpha Stores can be found fairly easily, most are pretty small but overflowing with an abundance of supplies.