You can order the books from the publisher’s homepage: Klear Textbooks or buy online from Coupang:
Integrated Korean: Beginning Level 1
Integrated Korean: Beginning Level 2
Integrated Korean: Intermediate Level 1
Integrated Korean: Intermediate Level 2
Integrated Korean Advanced Intermediate 1
Integrated Korean Advanced Intermediate 2
Integrated Korean: Advanced Level 1
Integrated Korean: Advanced Level 2
Integrated Korean: High Advanced Level 1
Integrated Korean: High Advanced Level 2
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You may also get a copy at Integrated Korean Intermediate 1 & 2 PDF (1st edition)