indiana university will sell to educators, but also you can try carolina biological -just remember that these animals must not be 'released' into the wild when your class is done with them, though considered a grand gesture it is both illegal and immoral to release non native species. if you live in a location that has native species you might be able to collect an egg mass and raise them till they hatch or morph (which ever is your desired time frame) for this you will need to contact your local wildlife office and discuss the matter with them-the rules vary from state to state....
