TheiSpot.com is where top-end illustrators work in many instances. It's more high-end than the average stock place. You can email artist there to discuss projects. The emails are in their portfolio/profile pages.
TheiSpot is sort of an agency and stock site combined.
You can also try the Society of Illustrators which is kind of like the Graphic Artist's Guild, but, well, for Illustrators.
Then there are agencies...
- https://www.lemonadeillustration.com/
- https://centralillustration.com/
- More with a google search for "illustration agency"
Whether or not an agency is more beneficial to an artist will really depend upon the agency. And, realistically, there's no way to know unless you or someone you know has worked with them.
To me, anyone working via crowdsourcing sites (Fiver, etc) has already determined that they feel there is enough benefit for them there. There's no need to feel like they are at a disadvantage. If that's where they've chosen to sell, they are fine with the site's terms.
You can also view work on Behance or Dribble, and contact the artist. Dribble even has a "Hire Me" button on artist pages.