Most of the popular 2d artists (if you go onto F95's main page and click on Comics & Stills you'll find some of them) there's also E-Hentai.com in which these same artists don't mind people posting their work which can be freely downloaded, seem easy going. But they seem to work on their own stuff, unless (as you say) someones willing to ante up some serious money for commissions.
The one site maybe developers can seek an artist(s), who's maybe looking for a few projects in order to get his or her name out there is Deviant Arts web site, but that would take time, time that wouldn't be used to continue developing the game. Or time the developer would rather use to take time for themselves.
Having said that, the other thing developers could do, (and I know supporters of games expect some type of favor from the developer) would be to ask fans/supporters to keep an eye out for artists.
Yeah, I kind of went through all the phases. Thankfully the new artist was recommended to me by my old one. We are also all italians, which helps a lot when describing the commission (I have had some real struggles with people who accept commissions in english, but can't really speak it).
That's why I am also very reluctant to search for new artists now. The ones I worked with for long are reliable and I have seen how things can go wrong (probably the worst one was the first guy I tried to hire for pixel art, english was his first language, but I still felt like we were aliens or something). As I mentioned, though, I don't want to make it sound all terrible, it's just that I have to see and handle things from my end, and that colours my perspective.
Anyway, art is the face of the game, but unless you really have a huge budget and a proper team, it can be hard to manage. Heck, even now it takes time for me to think of the scene, prepare some references and present all to the artist, and then there are further reviews during the various stages, so even when things go smoothly, there is a time commitment which increases with the number of commissions. Well, these are the challenges for a scrappy dev like me, it's all part of the job.