If it is not a standard sized window or size, the Mat it is going to cost you. Some of the places have a minimum, I think Michaels was around $20 and I asked even if it was just 5x7and they told me it was, so take that for what its worth. All of the standard sizes you can usually buy right off the shelf at said chain stores and those won't cost you much.
I Mat everything myself, it doesn't take me very long now. Soon once I get some space finished for all this stuff I won't have to break it all down when I finish. I have bought sheets 36"x40" (maybe even alittle longer) and it costs me less than 10 dollars. 11x14 uncut I think was 2 bucks.