With 3D models you pretty much get what you pay for. A high quality model takes a lot of work and most people don't like giving that much work away for free. Most of the time the free models are going to be fairly low res/low poly models. That said, if you're using a more professional 3D app there are ways to clean them up, increase their resolution, and make them more useful.
One of the better places you can check out is
TurboSquid. They have models at all price levels, including free. And sometimes you can find free or low cost models that are decent quality.