Image from Naver Maps
Let's face it. This is another one of those things you're not going to find in Chuncheon - Halloween costumes for adults. Since this is the case, the best bet is to go to a party store in Seoul, or more particularly Joy Party. There were other places I read about elsewhere online, but this information I got from a friend who lives in Seoul. I recommend checking this place out if you can because they have a good selection. They not only have Halloween costumes, but pranks as well. I would post pictures, but I forgot to bring my camera. Instead, I have a screenshot from Street View from Naver Maps.
The name of this place is Joy Party. It's in Gangnam-gu on Line 7, Exit 2. You should check this place out.
As a side note, I would like to point out that on my way there, the bus had some trouble and had to pull over. The bus driver was telling us that we had to wait for an empty bus to pick us up. Meanwhile, an angry ajossi was yelling at the bus driver. The bus driver told him to sit down. While he did sit down, he stood back up to continue yelling at him ... hilarious.