I just remembered that some of the local PDs here pass out free whistles to anyone who wants one. Maybe you could get in touch with them. They'll most likely be cheapo pea whistles but then again this is more about teaching than actual survival.

Also, contact your local CERT or Civil Air Patrol organizer and see if you can get some donations from them. I know my mother got a whole bag of freebies when she volunteered a couple years back.