I would go with the S.O.L. Survive Outdoors Longer Rescue Howler Whistle. The Howler Whistle is a dual-frequency 110-decibel rescue whistle that can be heard over a mile away in certain terrain.
SOL makes a lot of good survival gear, here's the details on their Rescue Howler Whistle:
-- Made with a dual-chamber, fail-safe, pea-less design, this whistle works in all weather conditions and won't freeze or stick to lips when cold.
-- Made from unbreakable ABS plastic, this whistle is dependable for a lifetime of trouble-free use.
-- Keep this whistle with you on all of your outdoor adventures and clip to a zipper, wear on a lanyard, or slip into a pocket for quick access in unexpected situations.Each pack contains two rescue whistles.
-- Triple-frequency sound cuts through surrounding noise.