Honestly, this forum is a really solid place to start your search.
You could also try places like r/ComicBookCollabs, r/lineWebtoon, or r/webtoonCanvas on reddit, but most of the webtoon subreddits are more about existing comics and self-promotion than finding people to work with.
r/ComicBookCollabs would be your best bet on reddit, but the focus is often more indie/print/kickstarter comics than webtoons. Plus even if you're an artist looking for a writer, making a solid post advertising yourself properly can be difficult when there's such a range of creators on there.
I would say try making a post here on the forums first, and then if you can't find anyone who clicks here, then try elsewhere afterwards.
