Where to buy good art books and t shirts?

I am visiting Korea very soon and I wanted to ask for a couple of recommendations

Do you know any places in Seoul/Busan where I can find artbooks by Korean illustrators? I'm aware of some of the bigger bookstores (K yobo, Starfield) and will definitely check those out, but I'm also looking for independent and left of field stores as well.

I'm also looking for any interesting street wear brands that make shirts and t-shirts with cool and interesting prints, as well as shops that sell them. A couple of brands I've got written down are thisisneverthat and Addererror, Stance, Mahagrid. So far, I'm not too keen on then but I'm wiling to check them out in person.

To give you a reference, I'm looking for something a little louder, like this https://dragonmade8.com/collections/shirts/Shirts

And this

https://www.thecriticalslidesociety.com/collections/tshirts/products/remains-tee?variant=45325122633897

Many thanks in advance!