I bought mats at both Walmart and Michaels Craft store. They are plain white mats, and are actually designed to have the signatures on them. For pens, I just took dark colored sharpies. Pillow cases I have never done, so not sure there.
The pillowcases turned out terrific! We bought Sharpie FABRIC markers It has been a year and the pillowcases have been through multiple washings and still look great.