Suggestion: Pet stores will have them.
Two clicks and your class will hear that you need their attention. Better than using your voice, this sturdy little clicker will cut through normal class noise and gather up everyone's attention.
Teacher Tip: Go to Michael's Arts & Crafts and get a roll of self-adhesive magnets. (These can be put to use in all kinds of ways in the classroom.)
By sticking a magnet to the backside of the clicker, you'll be able to store it easily on a magnetic white board.