@kjsharp: Although I've never actually started a custom set design project, I have done a bit of preliminary art sourcing in case I ever do. What has worked for me is plugging on-theme keywords into an image search, picking some images that have the desired aesthetics, then researching those artists. Without a big budget to commission specific styles and depictions, I've found the most reasonable way to maintain a cohesive artmosphere is to stick to just a handful of artists. It makes crediting them on card renders much more streamlined, too. For this to work, you have to find pretty prolific artists, of course.
Another option is to Google reverse image search a piece of art that exemplifies the feel you're looking for, and dig around in the "visually similar images" section. Finding the correct artist credits can often be a pain, so this method is tedious, and a bit hit-or-miss. But it does provide a finer control over the art style of your set.
Another option is to Google reverse image search a piece of art that exemplifies the feel you're looking for, and dig around in the "visually similar images" section. Finding the correct artist credits can often be a pain, so this method is tedious, and a bit hit-or-miss. But it does provide a finer control over the art style of your set.
Also, have a gander at this very old MTGS thread on the topic: http://www.mtgsalvation.com/forums/community-forums/creativity/custom-card-creation/368159-fantasy-art-thread
G'luck!