It is very hard for most creative types to find someone to collaborate with if they aren't already friends or acquainted, because most creative endeavors require a massive amount of time and energy and passion for the project, especially when there's no budget involved, and most of the time when you meet someone in a forum or a Discord server, they're not going to share the same passion for your project that you do.
In terms of comics, these forums are constantly inundated with writers looking for artists, expecting the artist to work for free and turn out pages purely for the sake of the passion of the story. But the artist doesn't share the same passion for the story that the writer does, and it takes so much more time for an artist to draw a page than it does for a writer to type up a page of script that most writers are left wanting. A script that takes someone two weeks to write can take a year to illustrate as a comic - that is a massive investment of that artist's time for nothing in return.
The best way to foster a collaboration with an artist is to have a budget. It seems a little cold and calculated, but it's the fact of life. If an artist wants to draw a comic or create artwork for the sake of passion of the craft, they're doing to draw their own comic and create their own art. If you really want to expect people to collaborate with you on a long-term project that is going to require weeks, months, or years of their time, you need to be prepared to pay them.
That being said, places like these forums are a good place to look for artists. There's a lot of cross-over between comic artists, illustrators, character designers, and concept artists. You can also follow artists you like on social media and ask them if they're be interested in doing commissions or work-for-hire, you can check the Digital Webbing forums, Behance, the deviantART job postings... I would avoid Fiverr as much as possible personally, it's detrimental to the quality of the final work and undermining the worth of an artist's time.
But, again, you're going to find success when you can pay an artist for their work. If you can't afford to pay an artist for their time and effort, your next best chance is to work with a friend or classmate or acquaintance who already likes you, respects your vision, knows they can work with you, and shares your passion for your project.