Children's slide for standard, manual type chart projectors. Lombart sells these for $72 (with a near vision card which I don't have). Great condition- slip it in and you're all set for the little ones! Shipping cost is additional.
http://lombartinstrument.com/product.php?productid=16392&cat=296&page=1
http://lombartinstrument.com/product.php?productid=16392&cat=296&page=1