Chalk pens work great, just like Steve has said. As for where to buy them, it looks like you can get them in the craft section of any store that would have one. I got mine at Walmart (I forget the exact price, but it was less than $5).
Once dry, it works just fine. Never smeared, smudged or wiped off during the battle or when folded and being transported to and from the game store. Wiped off with a dry paper towel just fine, no evidence it was ever there.
You do have to wait for it to dry, since it will smudge very easily when it is first applied (It looks and feels like wet paint), so you'll probably need to apply it ahead of time before you plan to use it.