I googled a little bit because I was curious (sounds like a great craft idea). Here is an article that is interesting:

http://www.artontiles.com/Paintingontiles.html

A ceramic shop would be a great place for ideas in general. You could maybe take the kiddos in there and have a painting day with some of their materials. It seems to me that when I was in one last summer, they even had some tiles or something you could use as coasters.