Extensive reading or reading for pleasure is an excellent way of getting a lot of "comprehensible input" in a foreign language. Graded readers provide this type of comprehensible input.
The website How to Study Korean offers a few graded readers for Korean (as PDF). The level of these readers is not entirely clear; you need to have studied 50 – 75 of the lessons on their website.
The blog post DIY Graded Korean Readers mentions the book
유머 영어 (Humor English) published by Kyobo but it is not clear from the blog post what its reading level is. (The publisher's website is in Korean only.)
The blog post Late December-February Book Haul contains pictures of books that are described as graded readers but their titles are not provided in the text. However, based on the descriptions, some of these books may actually be basal readers for Korean children.