You can get an Otterbox for about 20 bucks on Amazon. I've accidently slammed mine onto a tile floor with no damage whatsoever to case or iPod. The best part is that the screen protector is built into the case itself. It's not water-proof but it's protected very well from splashing and spills. I've also been very rough on the screen and so far zero scratches which is amazing for something made of soft plastic.
The one small bad part is that the Otterbox for iPod Touch was made for the 2nd Gen so the light sensor on the 3rd Gen's is covered. The only thing is that you won't have the auto-brightness feature. Not a big deal as far as I'm concerned for such robust protection.
There might be an issue with docking while inside this case but the hole is large enough for most brands of connectors. The hole for the headphone jack is only just large enough for the Apple headphones though.
I believe they sell an optional belt holster & arm strap too.
http://www.otterbox.com