@scriptandscribble,
It can be whatever type of paper you wish to use from a 24# bond to a stiffer Strathmore (different weights according to need/desire) to construction(a.k.a. sugar) paper, to onionskin to a coated/laminated stock. They can be a solid sheet or you can weave pieces of paper together forming a square or rectangular mat. Imagery can also be placed upon the surface ranging from a simple design to something slightly fun and functional with trivia questions or crossword puzzles or other games.
To add some style, instead of a plain edge perhaps a deckle border and might I suggest paper of a color other than just white? Sure using a goldenrod or canary or magenta might run a few extra pennies, but aren't you worth it?
Or just rip open a paper sack from the supermarket and use that. In other words,
any paper is place mat paper.