Pyle-Pro PMP30 Professional Megaphone/Bullhorn with Siren for $15.11.  Shipping is free on orders over $25.  If needed, use this Filler Tool to qualify for free shipping.  Here is a screenshot for your reference.  It features 800 yard range, variable volume control, and uses 4 x D batteries (not included).  Most places have it for about $24.