From what I understand they should be able to cut it in the store for free. Do you really need to have it cut? Also, ceramic or slate tiles are probably your best bet. I got ceramic tiles that were not ornate or anything at $0.40 a piece and they are 6"x6" each. Cost me like $10.00 or so get all the tiles for a 75 gallon tank. I just set them in loosely as I didn't want them stuck into the tank for cleaning. They hold heat amazingly well which makes it so easy to create a temperature gradient and a basking spot.
But again, they should be able to cut for you in the store for free. Even if they don't do it for free I don't believe it would cost much since it isn't like you're getting and cutting 100-250 tiles.