Hmm, they are a hard thing to track down. THE best are generally professionals and are either not open for commissions or are extremely expensive (obviously). There are some very good artists works on some VNs now but I'm presuming they are taken/busy like by Arsenixc and Icephei.There is Owen http://owen-c.deviantart.com/
who does great 3D work. I know he worked on Songs of Ariah and is/was working on Errant Heart too. These guys have been hired by Komi-Tsuku before (you could also ask him if he knows anyone) http://falynevarger.deviantart.com/
. After that you could possibly hit up Jack and see if he has any recomendations for people that he might not be currently working with.