As some one who ventured a bit into the creation of art, I would really advise you to get in touch with one rather than just "buying" models.
Art from different sources rarely work together (unless of course, they are meant to be) and high quality models alone won't make good graphics, everything needs to be put together with a sense of aesthetics to really convey a sense of quality. Just like starting writing a game without really having a idea how it should be structured doesn't really work just putting some graphics together won't really work either. It really requires expertise. Not just expertise in modelling/painting/pixeling but also in style and aesthetics.
So if you already know someone who is adept in digital art approach him, if not you might want to lurk somewhere (for example some IRC-Channel of some online community) with some artists.
If you have someone ask him about your game, ask what he thinks, ask how he would approach it from a art perspective. You might ask him whether he could do some graphics for it. Just make sure he is a part of the project, and not just some work slave.