That's a good answer. For art rather than design work: First, the main art buyers have massive pools of artists who want to work for them and apply - just like programmers. In some of these fields, one of the jobs of the art director is to keep an eye on who is around and how they are coming along in their skills. (Smaller buyers of art, like marketing agencies have built a network of freelancers). Second, many artists are terrible at business. Third you were looking for one-off work, not to hire an artist. So there is the issue of style. Some artists will work in any style but not many. So you look for images you like, and you talk to the artist.

And then because "terrible at business" you try to manage deliverables, ownership, timeframes, style again, unexpected insertion of political messages, etc, etc. Not impossible but likely none of this what you expected - until you put it on paper. There are art contract models out there.

There are forums where artists in specific narrow niches will post commission sheets and such - but they are niche forums for these niche interests. Just finding the forums (perhaps on discord) can be VERY difficult. See again "terrible at business".