Eeeh? Any illustrator ever? More or less?
Find an artist you like, check if their commissions are open, order what you need. Full illustrations aren't cheap however, especially if you're going to demand revisions if you aren't happy with something. Most novices are probably going to charge around $80-100* and the prices increase with the skill of the illustrator and depends on the purpose of the art. Book covers for example, can easily exceed $3000-5000.
*Depends on how many revisions you demand, of course. And if you just want line art for example, the price will drop.
Edit
You can also team up with people who aren't able to charge for their stuff yet. However, WC3 mods doesn't really translate to a whole lot of exposure these days, so you'll probably have to dig for a while before you find someone willing to do it.