Black tourmaline is a great choice, in fact all tourmaline will give off negative ions, so if you want something a bit prettier it will still benefit you a great deal. My wife is crazy about tourmaline and we have a few nice specimens, my favorite is a 30 lbs quartz with pink tourmaline.
Black quartz is another one you might want to look at, has some similar benefits to black tourmaline. I have a really nice 15 lbs black quartz in my bedroom. IMO black quartz is a bit more interesting because it usually has some clear and lighter tones mixed in with the darker areas, giving a nice contrast. Of course most of my opinions come from someone that collects for the beauty of the stones and not the metaphysical aspects.
You can get both types of stones really cheap on Ebay from reputable sellers. You could get 10-30 LBS specimens for $ 200-300 dollars.