8.5 is not even big at all! I wore that in middle school. ㅠㅠ
Unfortunately the best place to get cheap big size shoes is online. There are some stores (there’s one near Ewha Women’s University) that sell big size shoes but in reality, they didn’t have any pair I wanted in my size, and they said it would take one week to mail them to me. I couldn’t even try what I wanted on. The basement in Doota marks shoes up at least 50% what they are online. I bought a pair there for 60,000 and saw them online for 20,000 a few weeks later (ugh.) There are big size shoe stores in Itaewon but I don’t think they sell Korean shoes, just imported Western shoes that seem to be a few years old and are out of style and also very expensive (that’s what I’ve read online, at least.)
You might be able to find your size in stores because an 8.5 is between a 250 and 255 in Korea. All regular stores should carry up to size 250 and some might have 255.
Also, sad newsflash: if you wear bigger size shoes, they do not make cute shoes in your size. People with big feet are hideous giants who do not like to look cute, apparently. Nothing with lots of bows and sparkles and cute stuff comes in our size. If you are looking to buy some kind of special Korean shoes, I don’t think you will be able to find any. Unless you can fit into a regular size shoe!