I've tried hiring artists for video games, and the quotes from Upwork/Freelancer.com were about double the going rates (I understand that the platform fee of 20% contributes to part of that). I've had much better success with free, community-run forums instead:

https://polycount.com/categories/artist-looking-for-work

https://www.gamejobforum.com/

https://www.reddit.com/r/gameDevClassifieds/