We got our last batch at Office Depot; they tend to price competitively with Staples and vice versa.
But on a hunch I just searched Ikea and yes indeed they have them there too (but they call it "floor protector" as opposed to "chair mat"). Plus you get to call it a "Kolon" . Cheapter than the same size at Office Depot, but of course that's not good if you don't have an Ikea near you.
Doesn't seem like the kind of thing Walmart would have, but I could be wrong about that.