Make your own Sonic Grenades for dirt cheap... No guarantee if these things are as good as the Sonic Grenade a reviewed a few days back, but that was brilliant! At least, from the tormented expressions on other people's faces, I take it that it must have been. I stood there with my dandy little earplugs in, whistling away. Not that anybody could hear me from the pained looks on their faces and the shrill shrieking of the rubber grenade. Any way, if you wanted to hear all that, you could have just read the review.
But see if you can make your own for under a dollar. That's about 50p, but pricing will mean it costs more here in the UK. Don't let me start rambling on, just visit the how-to guide.