[NOTE: THIS IS NOT A SOLICITATION FOR APPLICANTS.]
I'm a graphic designer in the fortunate position of having enough work that I'm looking to hire an illustrator for some overflow work. Finding a service (like Fiverr or Upwork) in Google is easy enough, but I'm interested in providing the most benefit to the actual artist. Which aggregation services out there provide the highest return to their artists? While this isn't specifically a graphic design question, I suspect it can really only be answered by graphic artists who have experience with these kinds of services.
To be clear, I'm not talking about stock sites. I mean hiring for specific contract illustration work for instruction manuals.
[Follow-up] My hope with this question was to find out which illustration agencies and/or services are most beneficial to the artists. I am not asking for help finding agencies and services in general. I appreciate the responses, but none of the answers so far (Monday, April 22, 2019) address the question as asked.
[Example:] Illsutrationweb.us is the online presence of Illustration Ltd which has been around since the 1920's. They use agents around the world to represent their artists and help negotiate fees until an agreement is met, after which the client and the artist are put in direct contact to work out specifics. On the other hand, upwork.com creates a competitive price war atmosphere, where each artist's hourly rate seems to be the main focus, rather than their skill.