Yeah, I think in most cases folks hire their artists. I know I hired my artist. I always spend hours (sometimes days) combing through folks portfolios on various websites. In the past it was deviant art, but it was miserable having to sort through 100 furry inflation renders before finally finding a competent artist that doesn't draw fetish. Even then, many artists are busy or just may expect differing amounts of compensation for their work, so even after filtering the fetish garbage out, that doesn't mean I found an artist for my project, it just means I found someone to contact.
This past time I used Pixiv, which is where I found my current artist, Somon
. She's a real sweetheart.
I just reach out to the folks, giving them a brief description of what my project is and what I need them for and give them a budget. Either they say yay or nay, if it's a nay I move on to the next.
Since you will be going the route of volunteers, I think most volunteers are likely going to be folks who are looking to either gain more experience (in other words, still in the infancy of their creative hobby) or folks hoping for more exposure (but also still relatively early in their creative endeavor). In rare cases you may have an experienced individual offer to help but that's rare. My main point with this is that if you go the volunteer route, you don't get many choices for art style, limited to only the few who offer to assist you. Secondly, the problem with volunteers is that they are working for free and may not be able to commit to the project the same way as you do. So expect volunteers to come and go. If they do commit to your project, then they may be slow to turn around work (though, then again, even if you hire someone, depending on their workflow, you might still wind up with slow turn around)
The problem with volunteers coming and going is that it can really knock the wind out of the project because you are constantly having to slow down work to spend time seeking new volunteers. With art, you are unlikely to get someone skilled enough to emulate another person's art style, and therefore any time you have to seek out a new artist you will either have to accept having clashing styles or spend more time having old work redone in the new style.
This isn't to discourage you, I'm just speaking from experience and want to help you know what you are in for. I think generally it's better to find some way to save up money and commission someone. Money (generally, but not always) makes the whole operation go smoother.