Hi Gulliamo,
I bought a 10 pack of those whistles a couple of years ago at a flea market for $10.00.
I think they are called "buffalo whistles" and they are very loud, not quite as loud as a Fox 40 but close with a lower pitch I think.
The nice thing is that you can cut then down to about half the size and performance is not effected, this is what I did to fit them in my Altoid Kits.
Metal whistles are but good and bad. They are tough, but stick to your lips in the cold, I wrapped yellow electrical tape over the mouthpiece to prevent this, I also tied on a Spiderwire lanyard (I had cut off the rear ring section).
The whistles I have are not all aluminum, the small divider glued inside the whistle is plastic (about 1 cm in from the end to the cut-out), so it wlll burn if you subject it to a lot of heat.
I modded some of these whistles up, if I can figure out my son's digital camera I will post them.
Hope this helps,
Mike