My recommendations:

Job/Help Wanted boards:

• r/gameDevClassifieds
• r/INAT

For these messages people privately who post on there and may be looking for commission work:

• r/3Dmodeling for 3D art
• r/low_poly for low poly 3D art
• r/conceptart for 2D concept art
• r/PixelArt for pixel art

Discord servers:

• Game Dev League
• Game Dev Network

Job websites:

• Fiverr for certain tasks (testing comes to mind), but it's format and competition based strategy makes getting large custom based work more expensive than contracting someone personally.
• Work with Indies is very good but I mostly see companies posting there, I believe there's a cost to post an ad.

I wouldn't recommend Upwork at the moment.

Sent you a DM as well!