One bennefit to getting a standard office mat is they are specifically made for carpet because they have plactic "nubs" on the back that hold the weight of the mat and the chair more evenly and makes it easier to move the chair. The one I have is a "cheapie" that is meant for a low pile carpet, I'm going to replace it this year when the back to school sales start.
The prices usually depend on the size of the mat and weight of the plastics. Lots of cheap ones out there, but I'd invest a little more than that for something that will hold up. You should be able to find them at a local office supply store or walmart. If you are going to have it shippled you might also check prices on Amazon.