McNeil sells good mats at a fair price (I believe ~$80 for a 4 mat high quality rubber set). You can find their ads in any big circulation auto magazine, such as Car & Driver, Automobile, Motortrend, etc.
I have used some beefy rubber mats from my local auto parts store (Trak Auto) that runs approximately $20 a mat. Since I rarely to never have backseat passengers, I just bought them for the front. They work VERY well.