Not to be rude or sound pushy but I have a little experience with requesting resources. I don't have any big game titles and I'm not famous so my name is not significant. With that said, I usually have to offer money to get the attention of a potential graphic artist. Most of my stuff here on OGA was commissioned work. None of them cost too significantly on their own. If you can work up some specific tiles you need, specific diminsions (if tile size isn't already set), and contact some possible artists to ask about price you might get better (and much faster) results.

I chose to work with a style that's pretty common on OGA (Liberated Pixel Cup) so a lot of people enjoy seeing new updates and are willing to contribute even without profit. That approach doesn't always work for smaller projects/smaller sets of work.